It depends on your preference. Each have pros and cons. While each have about the same damage and penetration against other armored units, the M10 is much faster, and can be used very effectively in pairs or groups to swarm axis armor and quickly bring it down. It's armor is weak, and you have to pay a lot of attention microing it around so you don't lose it carelessly. The M10 is essentially useless against infantry units.

The Hellcat seems to be slightly more durable (I could be imagining this). It's considerably slower than the M10, but can camouflage itself when stationary. It also has a .50 cal turret MG, which makes it effective in dealing with infantry units.

Personally, I prefer the M10. It's a super micro-intensive unit, but when you are dealing with a lot of axis armor it's a great unit to do hit-and-runs against it. I like to mix in Shermans, or put BARs on my Riflemen, to deal with the infantry.

Of course, the M10 has always been one of my favorite units, and I haven't gotten to play around with the Hellcat a whole lot yet.