RS pro will have lower on resistance and in a spec class it should out perform the RSX with more power through put and is lighter (plastic case).
The issue is you could thermal the ESC if you trying put too much sustained power though it like a low turn mod for an offroad race. It is all about picking the right tool for the job. If you run multiple classes with a car (mod for club racing stock for big races or something like that) and don't want to swap out stuff error on the RSX side to make sure it will handle whatever you want to toss at it for power. I have RS pro in my 3 offroad 13.5 cars (4wd, 2wd buggy and ST) and it runs great.