The tank destroyers look like tanks but they have an open top turret and fewer machine guns. The M18 was fast but only had a half inch of armor and a 76mm gun. Its light weight meant it could cross ground that would bog down other vehicles like an M4 Sherman tank.
The M36 had more armor and a 90mm gun that would be better if you had to shoot at the thicker armor on the front of of German panzers. The extra armor would only protect against the lightest German anti tank guns but could be a big deal against indirect fire artillery.
M18 Hellcat - Wikipedia
M36 tank destroyer - Wikipedia