2 motors on each side for a dual-differential, as you name it, is probably so that there's more torque than just one motor. Using 4 wheels over 2 driven and a caster is more for stability than anything else I think..
Tank treads are useful when the terrain you're traveling on is bumpy, or not totally solid, like sand and gravel. The treads give extra traction and helps reduce the chance that the robot is stuck on a bump and the wheels don't contact the ground, which can happen with wheeled robots..
I guess when you compare tank treads vs dual differential, the first difference I can think of is that treads are very difficult to build, vs wheels which are more easily attached. Tank treads also make the robot a lot harder to turn, because of the extra traction. A regular differential (2 wheels and a caster) is the easiest to turn, followed by the dual-differential, then tank treads.