The EiRR Hellcat is vastly different to the vCoH Hellcat and has been for quite a while now.
The latest update reserves the cloak ability for vet 1 troops only with the exception of FJ and Snipers. My question is; is there any reason to use an M18 if it can't cloak?
Plenty of reason.
The M18 is now expensive for what? Marginally better armor and slower speed? It's not like it's going to be standing up to Panthers anyway, so is there a point to pay more without cloak? I guess the M18 can equip an MG, as if that makes a difference. Lowering the fuel cost (slightly) really didn't make a difference for any non-Armor companies because manpower and munitions are obviously the more useful resources for typical companies.
Armour is identical to the M10 but the M18 has +100 health, where the two units differentiate is that the Hellcat has a MUCH better gun for penetration.
By much better, I mean the M10 has 0.4 penetration vs Panther front whereas the Hellcat has 0.8, this means the M10 has 0.336 at long range vs the Hellcat's 0.672.
To break it down, here are the penetration modifiers (Range mods are 0.84 long, 0.92 medium, 1 short):
Panther - M18 0.8 / M10 0.4
Tiger - M18 0.85 / M10 0.5
StuG - M18 0.9 / M10 0.6
Hetzer - M18 0.9 / M10 0.6
Jagdpanther - M18 0.72 / M10 0.42
PzIV - M18 1.39 / M10 1.09 (M10 is very unlikely, but CAN bounce at max range)
All other unmodified armour types are penetrated by both tanks consistently.
Worth noting that the M10 has 4.2 reload vs the M18's 5.0, this means that if you're expecting heavy armour, bring Hellcats. If you're expecting medium/light armour, you'll be better off with M10's due to reload and the little extra speed.
The M10 on the other hand seems a lot more capable of avoiding taking damage all together with its speed, and I read it has better penetration (vCoH) on the CoH wiki. On top of this, it's cheaper.
FYI, CoH wiki is a very bad source of information for EiRR.
Any thoughts on how an m18 could be useful compared to an m10 guys?
It's plenty useful, it just isn't always the TD you'll necessarily want for a given situation. General rule of thumb? M10's for PE and M18's for WM. VERY general statement, but a fair one at a basic level.