For the Racer 2,
Front and rear suspension is a little different. Adjustable caster, camber and toe-in. The front belt tensioner is a little different than the one you can get for the RTR, you use a screw to adjust it. The fuel tank is supposed to be a proceed-type fuel tank. Car comes with MIP CVD's. The body mounts are a little different. Car comes with a better brake disk as well as a "racing clutch" Lightweight aluminum chassis. Upper deck is carbon fiber. Hingepins are slightly different also.
Parts are interchangable. The only downside is that if you want the better suspension, it will cost more because you'll need to buy the parts individually.
A week from now i'll be buying all the parts to do a Racer 2 suspension conversion.