The hellcat has 46 sight, 1 more sight then its weapon range--it spots for itself and can cloak. The recent patch buffed the hellcat's penetration so that it's just BARELY below that of the M10. The hellcat also tends to snipe men more often with its cannon. That being said, choosing it for the .50 cal is a bad choice, as you shouldnt be getting that close to units when you can kite them, and the fact that a TANK destroyer shouldnt be trying to take out a blob of tank busters. The hellcat's almost equal penetration, ability to cloak, and ability to spot for itself without having to invest in a jeep or other unit to spot for it lends it great synergy with british FFs. The M18 also synergises well with the M8, as the mines in addition to a cloaked ambush make for a great combination. In contrast, the M10 synergises the best with the T17, because the T17 can stun targets and allow a blob of M10s to swarm in and r4p3 whatever it is. Hard. The M18 can also 'scoot shoot' with its ambush, driving up to, stopping, camoing, shooting, and resuming moving on a target, essentially getting the ambush bonus and then retaining basic mobility. Its a hard tactic, but if, say, an enemy P4 is backing away, you can drive up, camo shot it, and then resume the chase while still staying out of the P4s range. Its a tough tactic, but well worth it.
The M10 and the M18 have the same armor types and HP counts (armor_tank_destroyer; 300, respectively) with the higher accel/decel of the M10 lending it to better crushing. The Hellcats oversized model also makes it hard to crush with, and also makes its pathing abysmal. In all frankness, the Hellcat is by far the better unit (lacks the misfire bug that STILL hasnt been patched in vcoh).