TL;DR?
Pick Motivity or Technique and max it out ASAP (~27 in a single stat) while sometimes dropping a few levels into Vitality, Vigor, or Capacity as needed. (If you use an Advance scaling weapon then 16 is the best stopping point for it) I'd strongly recommend AGAINST leveling BOTH Motivity AND Technique right at the start even as Path of Balance - I tried that in my playthough and found myself exp starved when I really needed more of the utility stats.

Methodology:
I created a character of each "class," took each one up to the 1st boss and then used Cheat Engine to give me a million Ergo so I could examine what happens to each stat as you put as many points into each one as possible (or at least up to level 30) I did this by hand entering all the changing values into a spreadsheet which you can access {HERE}[www.dropbox.com]

Before defeating the first boss there is a hard cap of 10 level ups, but past that point you could take any stat all the way to 100.

Physical / Fire / Electric / Acid DEF is mostly just tied to character level so putting a level in ANYTHING will see some gains (tho Vigor appears to give half the benefit)

HP gains start decreasing right after 10 with a harder decrease after 12 and it is just diminishing returns from there on.

Guard is linear until 13 with only a minor dip at 14 and then just diminishing returns after 15.

Recommendation: Everyone should take Vitality to 10, but probably not worth going further than 15.

If the Pulse Cell does actually scale with your maxHP then that does make Vitality more valueable, but going past 15 you'll still encounter steeper and steeper diminishing returns eg:
17 Vitality is the last time it'll give you +16 HP
19 Vitality is the last time it'll give you +15 HP
21 Vitality is the last time it'll give you +14 HP
23 Vitality is the last time it'll give you +13 HP
26 Vitality is the last time it'll give you +12 HP
29 Vitality is the last time it'll give you +11 HP
(all of this info has been in the spreadsheet since the start btw, but when I have time I'll try to map the exact healing values and see if any rounding errors produce new breakboints)

Stamina gains are only ever decreasing so even from 6, the more levels you put in? The less benefit you get from each additional level spent (with *some* useful cutoffs)

Eg putting 10 levels into Vigor as Path of Strength would result in 50 extra Stamina, doing the same as Path of Balance would return 43 extra Stamina, and as Path of Dexterity would only return 32 extra Stamina.

Stamina is a very useful thing to have... so much so that the devs punish you for seeking more of it by only giving you half the usual DEF you'd get from putting a level in anything else...

Recommendation: Some good stopping points are: 8, 11, 15, 21
Just use the stopping point that feels right based on your playstyle.

DISCLAIMER: Patch notes say these values were changed in patch 1.2 so the information in this section could be out-of-date!

Legion and Weight gains are almost perfectly linear for all values tested with a minor dip in Legion gains at 20 and improved minor Weight gains at 10, 14, 19, 25 (but literally just an extra 0.1 at those levels)

Weight Class: under 30% = LIGHT, under 60% = NORMAL

Recommendation: Take only as much as you need to remain in the Weight Class you want to be.
Also consider this a dump stat once you get everything else where you want it to be; can try to be LIGHT Weight Class even with all the best gear!

Both stats have identical progression; it is only which weapons favor which which will determine how you invest your levels. Eg C/C weapon? Both will contribute equally. B/D weapon? Invest in the one which has the B scaling.

Physical ATK gains are a little weird at the start, but they all INCREASE in value the more levels you put in so - pick one and put ALL your levels in based on which weapon you use... probably ASAP too at least up until ~24, then it finally starts seeing diminishing returns with a steeper decline after ~27.

This means you will get the best results from whatever class can reach the target value(s) with the stat(s) governing your main weapon soonest.
Motivity = Path of Strength
Technique = Path of Dexterity
BOTH = Path of Balance

The more levels spent on either stat BEFORE 13, the more levels that class is wasting.
Eg the Path of Balance would "waste" 6 levels getting both Motivity & Technique up to 13.
Path of Dexterity "wastes" 1 level getting Technique to 13 but would waste 7 levels trying to bring Motivity up to 13, for a total of 8 sub-optimal levels if you were to try leveling both.
Path of Strength is the same as Path of Dexterity, but just reversed for stats.

Recommendation: If you only use one stat take it to ~27, if you use BOTH then take them both to at least 24 but only level up ONE before switching to your utility stats and then come back for the 2nd or else you'll likely become exp starved as each level costs more and more.

Legion gains are again mostly linear just with rounding errors which makes the following levels see an extra point of benefit: 8, 10, 12, 15, 17, 19, 21, 23, 25, 28, 30 (that's as far as I tested)

Resistance gains start seeing diminishing returns as soon as lvl 8

Elemental Weapon ATK gains peak at 16 with steeper diminishing returns after 19, 21, 24

Recommendation: If you'd like a bit of a resistance buff you can drop a couple points into it and raise it to 8 (or not) but if you use an Advance scaling weapon then take it to at least 16.

Path of Balance
9 Vitality (+1 or +6)
7 Vigor (at least +1)
8 Capacity
9 Motivity (at least +15)
9 Technique (at least +15)
7 Advance (+9, if you use it)

Path of Dexterity
8 Vitality (+2 or +7)
12 Vigor
7 Capacity
5 Motivity
11 Technique (+16)
6 Advance (+10, if you use it)

Path of Strength
11 Vitality (+1, at most +4)
5 Vigor (at least +3)
11 Capacity
11 Motivity (+16)
5 Technique
6 Advance (+10, if you use it)

A note on variability and reproducible test results.
Equipment can change base values; eg using the +HP locket -> you'd see different values than those listed in the spreadsheet where that item was not equipped during testing, but the difference between levels should remain the same and thus the soft caps should remain the same as well.

Weapon ATK values were recorded from B scaling items; specifically the starting arm for Motivity and Technique values and the electric baton purchased from the vendor before the Mad Donkey boss for Advance values. It is possible the 16 / 24 / 27 soft caps could vary slightly with other weapons and different stat scalings, this is because of rounding errors - most games do not store decimal values for things like Weapon ATK and where a value might round up in one case, it could just as easily round down in another case but the differences should only be +/- 1.

If such differences bother you, just pay closer attention to how values change as you get close to the listed soft caps - eg first testing pass I saw +3 ATK at 27, at 28 it was also +3 and then at 29 it went up to +4 before decreasing to +3 at 30 and +2 at 31. If you see something different for a different weapon, do not feel required to match your stats to my test results - if the data is different under different conditions, you can stop at eg 28 or 30 instead based on the values you are seeing. But after further testing it looks more like the limits are at 24/27 rather than 23/29.
(check the spreadsheet linked at the start to see what I'm seeing for yourselves)

It is possible for different weapons to use completely different progression curves (usually game devs are kind enough NOT to do that) but if you think a weapon might be doing that then it could have completely different soft caps than what I saw in my testing; feel free to leave a comment below and I'll try to do a thorough analysis on that weapon as well.

Path of Strength is VERY well optimized if you want slow but heavy hitting
Path of Balance is well optimized if you will be investing in BOTH Motivity & Technique
Path of Dexterity is NOT as well optimized but speed *is* very effective

Path of Balance would probably be the most powerful at the end game but require more grinding to really make it shine. Path of Dexterity is probably the easiest to start out with or for a casual player.

If you found this guide helpful please remember to rate and favorite it!

Values were recorded with version 1.1.2.0 of the game