if its a buggy id stay .21 for racing.
from what i read, the RB motors are great
C5 - high end motor, revs forever, good low end with the proper clutch/pipe setup BUT it apparently is a gas hog. great motor for bashing or short races. you will have to pit too many times for a long a-main
Ws7II - more bottom end, still revs alot, great power, but sips fuel, much better for racing as it wont overpower the track. most everyones pick for racing.
all the larger CC motors are awesome, and i want one, but only for bashing, i wouldnt race one.
just look at who wins the races.
also if you dont care about fuel econo or racing RB has a .28 coming out soon thats supposed to rip.