The M18 would be a better fit, IMO. Here is why (going from memory, please forgive the lack of exact stats):
The M10 is very much like the medium tanks we already have save for the open top. It is big, slow, and has a cannon very similar in capabilities as the Pzr IV, T34, and to a lesser extent the Firefly M4. Its armor is not all that different either. It offers nothing we currently do already have.
The M18 is smaller (18 tons vs 34 ton M10) and faster (50mph vs 30mph M10), and has less armor (only .3in on sides and rear and 1in in front) than our current tanks. The M18 (or the M10) would be just as easy to knock out from the air as is an M16, Ost, or Whirble (actually more easy due to minimal air defense), and actually are very expose to HE fire as well. We would have something that is a legit "shoot-n-scoot" threat to other armor, but yet easy to knock out from the air and "easier" to knock out vs other armor due to a lighter armor if we had the M18. The M10 offers a much smaller, if any, addition to the game.
Edit: I forgot to add that the cannons have the same capability. I would also like to add that each of these tank destroyers fielded very few, if any, HE rounds so they would be anti-GV only.
One last thing: if we get the M18, I foresee a whole new set of tactics coming into play. The M18 simply can not get into a round vs round match with other tanks, it cant take the heat since it doesnt have the armor. It would have to use its speed and acceleration to shoot n move. It is almost as fast as the M8, but with a far superior cannon.