Rough Guide to Space Marines
Space Marines are an elite fraction. Most of their stuff is more expensive and more effective. They will be you infantry force late game, and unlike Steel Legion Infantry, they will do work. They have no superheavies, but their Land Raiders are incredibly potent and durable. Due to all 3 flavors having similar unit lists all three are in this. By far the best thing about space marines is their insane initiative, ensure they’ll strike first almost every time.

General rule of thumb is the Blood angels are the costliest, but have the best accuracy. Salamanders are a middle ground with higher durability but lower accuracy. Ultramarines are the cheapest but worst. They have the same accuracy as Salamanders, as tough as Blood Angels, few unique units and typically not worth it if the others are available.

Infantry
The thing you’ll use space marines mostly for. There are a large variety of them and all useful to some extent (unlike Steel Legion’s Hive units, which are useless unless role playing). They are very tough, deadly, fast and most can act as scouts or spotters.

Walkers
Dreadnaughts are expensive, tough, versatile and deadly. They cost as much as some Steel Legion superheavies, but they have good reason to.

Vehicles
Scouts scouts scouts. Ever want a scout? Have a scout. These scouts are the best you can get par the Warhound scout titan. All are incredibly fast, and there are some that are a bit worse at scouting, but with more guns to compensate.

Tanks
The average space marine tank is roughly on par with Leman Russ tanks, but the unit is better. Each unit has 7 tanks in the group compared to the Russ’s 5, and 5 HP compared to 3. The basically means they have more firepower as a unit, more durability per tank (so its easier to keep veterancy) and generally just better.

Aircraft
Space marine aircraft in similar vein to everything else are great. They have less durability than anything else space marine (even scouts have better unit durability) but their firepower is nothing to laugh at. Just make sure they won’t be swamped by multiple enemies, as that’s an easy way to lose a lot of money.

Command Squad (All)
-An very useful unit, decent ranged attack and a powerful melee attack.
-Since orks are good in melee the melee attack can sometimes backfire with both units doing significantly more damage.
-Leadership trait vital for helping active units keep their moral up. Keeping a command squad next to your titans (in campaign) will let you never have to rest and refit them, at the cost of only moving as fast as the command squad and limited positioning options.

Scout Squad (All)
-Marine version of Raltings, good snipers
-Cheap, but equally durable as steel legion infantry
-Cannot hit adjacent enemies, so they’re a ripe target for melee
-Tend to build exp very quickly

Assault Squad (All)
-Short range killers of everything not painted red
-Ideal for city fights as melee ingores cover
-Can kill medium armor in close combat if needed, but generally should stay away from nobs and meganobs

Tactical Squad (All)
-Standard Space Marine Infantry
-Effective vs medium or lower armor, good vs all infantry at range

Tactical Squad with Graviton Gun (All)
-Graviton Gun gives the unit good anti-tank ability
-Should be in cover when engaging heavy armor
-Can hurt superheavies if needed
-Significantly more costly than standard Tactical Marines, but overall more firepower for lower durability

Vanguard Veterans (All)
-Like assault marines, meant for close combat
-More expensive than Assault Marines and longer ranged shooting attack
-Much higher cost relative to assaukt marines for minor shooting and durability boost

Sternguard Veterans (All)
-Slightly more costly than Tactical Marines, slightly better in every way

Death Company Squad (Blood Angels)
-Much better stats than Sternguard Veterans with exception to weapons
-Very nasty and very good

Death Company Assault Squad (Blood Angels)
-Better than Assault Marines in almost every way

Sanguinary Guard (Blood Angels)
-They put the Death Guard to shame
-Close combat weapon can hurt anything and slaughter weaker enemies
-Just because they can hurt a stompa does NOT mean it’s always a good idea
-They still die a bit fast if the enemy attacks them, as they often do

Salamander Firedrakes (Salamanders)
-Very durable infantry unit (more durable unit than Leman Russ units)
-Short range devastating heavy flamer
-Excellent in city fights, although the enemy does try to kill them from range
-Vulnerable to heavy melee infantry

Devastator Squad with Heavy Bolters (All)
-Very effective against light infantry, struggles with armored foes
-Basically infantry version of the Leman Russ Punisher
-Low initiative means the enemy will always strike first unless its supporting an ally
-Will Support adjacent allies being attacked if in range
-Very expensive for Infantry

Devastator Squad with Multi-meltas (All)
-Excellent anti-medium armor
-Supports allies
-Low initiative
-Expensive

Terminator Squad (All)
-Ultimate heavy infantry
-Twice as durable as Leman Russ units
-Devastating in close combat
-Slow

Terminator Squad with Assault Cannon (All)
-Terminator Squad with significantly better anti-infantry at range

Terminator Squad with Cyclone(All)
-Gains powerful range 3 anti-tank missiles
-Melee weapon worse vs armor, but more attacks leading to overall more effectiveness
-Propably the most practical Terminator unit

Terminator Assault Squad (All)
-Terminators that can hurt superheavies easily in close combat
-Can be hurt from range w/o being able to retaliate

Centurion Squad with Heavy Bolters (Salamanders and Ultramarines)
-A better Devastator Squad with Heavy Bolters
-Much more deadly and durable
-Has low initiative than most Space Marines, but still higher than most orks
-No anti-tank ability
-Expensive

Centurion Squad with Grav Cannons (Salamanders and Ultramarines)
-Anti-tank version of Centurions
-Threat to any form of armor, including titans, although cover is very useful in such cases

Centurion Assault Squad with Flamers (Salamanders and Ultramarines)
-Deadly close combat variant
-Ranged attacks best vs infantry, but close combat weapons can kill superheavies

Centurion Assault Squad with Meltaguns (Salamanders and Ultramarines)
-Similar to squad with flamers, but better ranged attack vs armor but weaker vs infantry

Thunderfire Cannon (all)
-Powerful direct-fire artillery
-Cannot move and shoot
-As expensive as Basilisk, but shorter range
-Bit hard to make useful since by the time you get them you are mostly attacking rather than defending (campaign)
-Very fragile, will die in great numbers if attacked.

Dreadnought (All)
-Expensive jack of all trades killing machine
-Very durable
-Since its general purpose it doesn’t tend too kill as much as it should considering the price tag, but akways handy

Hellfire Dreadnought (All)
-Ranged anti-tank Dreadnought
-Effective vs medium armor or heavy armor, but should probably not fight titans

Ironclad Dreadnought (All)
-Very powerful close range dreadnought
-Best vs infantry, excellent in cities
-Since it’s a vehicle it can have trouble moving through cities

Venerable Dreadnought (Salamanders and Ultramarines)
-Most expensive dreadnought
-Has the leadership trait, which makes it very useful
-Slightly faster than a reagular dreadnought (in both initiative and movement)
-Same weapons/offensive ability as regular dreadnoughts

Furioso Dreadnought (Blood Angels)
-Nastier version of Ironclad Dreadnought for the slight cost increase

Bike Squad (All)
-Absurdly fast with good spotting distance makes this unit an good scout scout
-Decent firepower, but less durability compared to tactical squads so prolonged fights not recommended

Assault Bike Squad (All)
-Sacrifices scouting ability for anti-tank ability
-Best vs medium armor

Scout Bike Squadron (All)
-Even faster than normal biker squads and better spotting distance makes this the best pure scout
-Is pretty fragile and same cost as normal biker squads, not meant for fighting

Land Speeder (All)
-Same scouting ability as Bike Squad but worse firepower.
-Bit less durable than Bike Squads (less defense, more HP)

Land Speeder Tornado (All)
-Adds a Heavy Flamer to the Land Speeder, making it and anti-infantry threat
-Dies too fast to do much in most cases and expensive

Land Speeder Typhoon (All)
-Adds anti-tank missiles to the Land Speeder
-Fragile enough that engaging any armor is risky, but can work well as a finisher

Land Speeder Tempest (All)
-Replaces the Typhoon’s Heavy Bolter with an Assault Cannon
-Still fragile and expensive

Land Speeder Storm (Ultramarines)
-More expensive Land Speeder with no benefit I can see

Stalker (All)
-Space Marine equivalent of the Hydra
-More fire power and durability than the Hydra, but twice as expensive

Hunter (All)
-Another anti-air platform
-Immense range
-Lower shot count, so can be finicky

Predator Annihilator (All)
-Standard Space Marine tank
-Decent Firepower, better in most ways compared to Leman Russ tanks

Predator Annihilator (upgraded) (All)
-True Predator Annihilator lore wise
-Anti-tank tank, can take on most standard enemy armor, but can have accuracy issues at range
-Better initiative and durability

Predator Destructor (All)
-Good anti-infantry ability
-Same cost as Predator Annihilator

Predator Destructor (upgraded) (All)
-Better initiative and durability

Ball Predator (Blood Angels)
-Short range devastating anti-infantry tank
-Probably the best unit for anti-infantry, but only at range 1, so not always the best option

Razorback with Heavy Bolters (All)
-Cheap (for space marines) durable anti-infantry tank
-Bit lacking in firepower and cannot fight armor

Razorback with Lascannons (All)
-Cheap (for space marines) durable anti-tank tank
-Terrible anti-infantry ability

Damocles (All)
-Vehicle with Leadership
-Minor anti-infantry
-Can be a more mobile and durable way to keep leadership with your units
-Lackluster for any task but being a units with leadership or damage soak, venerable dreadnoughts or company command squads probably better choices overall

Whirlwind (All)
-Short range anti-infantry artillery

Vindicator (All)
-Marine version of the Thunderer
-Better than the Thunderer in most ways

Vindicator (upgraded) (All)
-Better initiative and durability

Land Raider (All)
-Now a transport for terminators
-Good ranged anti-tank, probably the best out of land raider units
-Losing 1 terminator loses a landraider, so can be very expensive to get and maintain, but massive mobility boost to the terminators

Land Raider Achilles (All)
-Long range powerful main cannon backed up by medium range anti-armor weapons
-Good vs all targets

Land Raider Crusader (All)
-Slaughter of Infantry
-Some decent anti-tank ability, best vs infantry targets though

Land Raider Helios (All)
-Same weapons as Predator Annihilator (upgraded) but with a Whirlwind Launcher and Land Raider Durability and lower shot count
-Effective vs all targets
-Missiles can hit BLOS

Land Raider Helios with Hyperios (All)
-Replaces the Whirlwind Launcher with an medium range anti-air launcher
-Considering the weakness of air units, this is not really needed

Land Raider Prometheus (All)
-Land Raider Heavy Bolter platform
-Bit of a waste, more expensive than normal Land Raider and cannot engage anything armored or kill infantry fast enough to justify the cost, although the range advantage over the other 2 anti-infantry landraiders will help

Land Raider Redeemer (All)
-Land Raider variant of a Hellhound, and even better
-Slaughters Infantry and can hurt medium-heavy armor
-Best land raider in an assault

Stormtalon (All)
-Expensive anti-aircraft flier
-Fragile, but long missile attack that is dangerous against both aircraft and fliers

Stormraven (All)
-Powerful expensive gunship
-Hurts armor or infantry easily

Storm Eagle (All)
-More powerful gunship
-Best vs armor but good vs all