Ok, went and did some math. Please forgive the poor formatting, just wrote it up in notepad. If you find any errors in my reasoning or calculation, please let me know. Note that I did all of my calculations (except for the blurb at the end) assuming that one would be fully buffed, have the 9% aim talent, and assuming a 70%/30% damage split tech/ranged.

Removed my helm from my dps set, and fully buffed without the helm I have 2190 Aim and 147 cunning.

My tanking helm has 113 aim, while the helm I had on Mako has 114 cunning. I figure this gives a slight edge to cunning, but it shouldn't matter as this won't be close anyway.

FYI: I am running the Assault/Pryo spec which takes the 9% aim talent (8/5/28). Numbers below will reflect AFTER 9% buff and all class buffs.

So, looking at Aim and cunning without the helm slot on.

Aim(2190):

Ranged Damage Bonus: +438

Ranged Crit Chance: +13.44%

Tech Dmg: +438

Tech Crit: +13.44%

Cunning(147):

Tech dmg: +29.4

Tech Crit: +1.17

Alright, so let's put the aim helm on, an increase of 113 aim (cunning stats will remain the same)

Aim is now at 2319 (a bit more than the 113 on the helm due to the sorc buff and the 9% aim talent)

Rng/tech Dmg: +463

Rng/tech Crit: +14.00%

So an overall increase of:

Rng/Tech Dmg: +25

Rng/Tech Crit: 0.56%

Alright, now with the cunning helm. Aim is back to 2190, cunning now at 267.

Cunning of 267 gives me:

Tech Dmg: +53.3

Tech Crit: +2.09%

Which is an increase over the base cunning value of:

Tech Dmg: 23.9

Tech Crit: +.92

Multiply that by .7 (because 30% of our damage as Assault/Pyro is ranged) and we get:

Damage: 16.73

Crit: .64%

So, by swapping out 113 aim in favor of 114 crit, we lose 8.27 bonus damage (equiavalent to about 36 power, fully buffed), and we gain .08% chance to crit (again, adjusted because tech is only 70% of our damage)

So, how much crit rating will give an equivalent amount of crit (remember, we should be modifying our stats so that crit and power are roughly equal, so I'm going to conver that .08% crit into crit rating.

My crit% from crit rating at 262 crit rating is 9.78. Dropping my matrix cube (27 crit rating) lowers this to 8.95 a change of .83% crit (again, only looking at the crit % from crit rating, not taking the change in aim into account) So by my calculations, the crit difference in cunning is worth about 1.79 crit rating.

TLDR: For fully buffed assault/pyro (with 3 points in 9% aim), trading 113 aim for 114 cunning is roughly equivalent to dropping 36 power for almost 2 crit.

Ok, so let's just assume somehow that 90% of our damage is tech (if we were playing something like a marksman sniper or Adv Prototype Ptech/Tactics VG) and see where the comparison brings us.

The cunning additions will now be equivalent to:

Damage: 21.51

Crit: +.83

So now there's a damage differene of 3.49 and a crit difference of .27%

Damage difference = about 15.19 power equivalent

Crit difference = 8.78 crit rating equivalent.

Still firmly in Aim's favor over cunning.