Vance: ,, well, I'm an airplane guy with armor knowledge.. lol.
Tank Destroyers,,,, well.. it depends on whose "doctrine" your following.
US Doctrine ,was hit em fast, hard, scoot away. Most US tank destroyers were upgunned from their standard tank brothers, M-10 had the 76 MM, M-36 90MM, Hellcat 76 MM, all were lighter armored than the main tanks, most were faster. The Hellcat can do over 45 MPH from what I remember..
Most German and Russian TD's were big gunned, heavy armored machines. not all, but most.. and it depends on where in the War you are, time wise... The Germans liked to mount big guns on refurbed chassis of battle worn tanks( rebuilt) They also built many purpose built TD's also, with heavy armor and big guns.. Most of these had no turret, they had a limited traverse mounted gun either mounted on a chassis, or built into a "box" like top..
Russians did the same on some, but also liked to build very big tanks with very big guns..
I'm sure others here have more and better info... ???...