The 12M is the newest car, it has an entirely new front suspension that is very adjustable, reactive caster, sway bar, etc. Also the car has a lower T-bar set up the is even with the cars chassis. This was designed to provide a lower roll center for tracks with less traction, this is adjustable too. The roll center can be raised up or down, you can also add anti sqat to the rear.
The G3 is the former top of the line Corally, and is still a good car. The G6 I would imagine is the G3 with the 6 cell chassis.