Town killer? the IS-2 was a Breakthrough heavy tank - used to bust through fortifications with he rounds, but its mobility was on par with a Tiger, Armor was far better and Gun packed a hell of a punch enough to knock out a Tiger/Panther. There is no inferior to it, IS-2 would be on par with a King Tiger.
How many King Tigers are relegated to Town killing? none.
Panzer III wouldn't get more use than a Churchill tank, KV-1 yes depending which model, KV-85 would get used more then both churchill and Panzer III, KV-1 on other hand with hand cranked turret and no mobility would be useless, other than for scenarios as the Panzer III. Churchill on the other hand has VERY good armor, while a bit slower, has a pretty decent gun and quite a few modifications from a Townkiller with 95mm gun, or 75mm, standard issue was a 6lb gun, early versions had a 2lb gun in the turret and 75mm Howitzer in the hull. Making it a pretty versatile tank all round, however if that doesn't take the case then Cromwell tank would certainly be added.
Cromwell did 40mph, had a 75mm long Barrel, and some 75mm or so of armor, pretty balanced medium tank setup, better then Panzer 4 - slightly less then M4(76) while faster then every tank even a T34/85.
Assuming HTC wants to go the route of making components for other things, the Panzer III makes a good choice - however as most will agree HTC tends to need time developing new things. Spending months to develop a tank nobody will use, just to have components for other things might not favor many players.
Especially when the Panzer III would pretty much be relegated to Scenario setups, since MW and EW are rarely populated by tankers.
IS-2 is 2 mph slower, has a worse HP/ton ratio, has a much slower firing weapon, poorer balistics, and little better penetration than the 8,8cm KwK 36 L/56. Its armor is going to be about that of the Panther from the front, minus the lower hull weak spot, and about as much as the Tiger I on the hull sides.
Really, its HE, and its frontal armor are going to be the only redeeming qualities, the 88mm L'56, 75mm L'70, and 17lber will out-shoot the A19, even if they won't out-hit it. And the 88mm L'71 will do both.
I don't know about you, but that sounds a hell of a lot like inferiority, and town-killer set up.
Few, because its 88mm L'71 can get off about 6 rounds per minute, as opposed to the IS-2's ~3ish, and its far and away the best anti-vehicle cannon in the game. Even the 128mm L'55 didn't match it for maximum penetration in real life.
Depending on the model, the Churchill would without question see less use than a Panzer III. 25mph, 50mm armor, and a 50mm L'60 is better than 15mph, 102mm of armor, and a 2lber. A Mk III or IV would see more use than the Panzer III J, but even the Ausf. L might be close to equal. A Mk VI would probably see less use, since its essentially an up-armored but significantly slower M4.
So yeah, it really comes down to the variant in question.
The Cromwell had a short 75mm, firing the same rounds as the US M3 Tank Gun.