Take your pick. Both cars are good out on the track. Ultimately, you are the deciding factor in your car's performance - not the cars themselves.
By the time you buy the universals (front/rear) and one-piece engine mount (you really need to get this) for the MTX-4, the price is about the same as the RRR. If you are going on price alone, it's the same.
It's much easier to install and remove the engine in the RRR, than it is in the MTX-4. This is the only issue I have with the car. Maybe I'm just spoiled because I own the RRR. Other issues I have with the car are minor.
Download the manual for both cars and take a look at how each one is put together. Although a lot of the parts look the same, the cars are engineered quite differently. Mechanically, each car has it's pros and cons from a wrenching perspective - but that doesn't really matter once the car is out on the track. It only matters when you have to fix something or do routine maintenance.
Hope this helps.
