If you're just beating around car parks the difference between the two cars is minimal, the extra money spent on something like the XXX-S is to get advantage when you're racing on the track.
Having said that, im not sure exactly what it is, but I was racing against a XXX-S RTR with my XXX-S G+ and my G+ was much much quicker everywhere in the corners and on the straights even though we had a similar motor (itsc stocks)..