B1 assasins is pretty good cause of high dmg sniper rifle with them and much more aggro distance(moment when they start aim and shoot) than B2 and Bxcommandos.
I still pref B2 army in tank role cause they armor is good, but with good backup from sniper enemies dying much faster.

OOM Security Battle Droid (Level 1 - STR: 6 AGI: 3 - 1H: 71 2H: 69 RIFLE: 82 PISTOL: 65)
OOM Marine Battle Droid (Level 12 - STR: 10 AGI: 6 - 1H: 152 2H: 157 RIFLE: 181 PISTOL: 163)
OOM Command Droid (Level 24 - STR: 16 AGI: 9 - 1H: 168 2H: 167 RIFLE: 182 PISTOL: 160)
OOM Pilot Battle Droid (Level 12 - STR: 10 AGI: 6 - 1H: 140 2H: 151 RIFLE: 160 PISTOL: 160)

B1-Series Battle Droid (Level 1 - STR: 6 AGI: 3 - 1H: 67 2H: 73 RIFLE: 125 PISTOL: 64)
B1-Series Assassin (Level 12 - STR: 10 AGI: 6 - 1H: 149 2H: 147 RIFLE: 161 PISTOL: 146)
BX-Series Commando (Level 24 - STR: 16 AGI: 9 - 1H: 174 2H: 166 RIFLE: 183 PISTOL: 170)

Looking at the troop tree (hit "show spoilers")[forums.taleworlds.com], the main reason to buy OOM series is that there is more variety. OOM series can eventually upgrade to pilot droids or elite combat troops (note that BX-series commando is roughly equivalent to OOM command droid). On the other hand, it looks like B1 droids are better armed or trained than their OOM cousins (this is especially true of the B1 assassin, which is armed with an incredibly strong rifle as opposed to an E-5. I wonder if weekly wages also play a part, but I can't check that immediately.

There aren't any available troop trees for the B2 series, but a quick glance at its stats in game show that it the regular B2 begins at level 25 (even better than the Command or Commando), with a presumably higher level for the C-B3.

I just spam b1 battle droids. Nothing says you aren't taking that planet like having thousands of droids in the garrison. It takes a while to get up to the point to where you can just build(recruit) droids in large quantities at a time.

ROGER ROGER

I've mostly used IG-Series Assasin Droids and they are totally worth the high cost.

IG series are great, but you can't really get a reliable source for them. You have to wait until they sloowwlllyyy trickle back into the mercenary bars. Their slow replenish is not fit for use as rank-and-file.