If you want to go for more power later on you should get the 35 gtx! It will spool up just as fast as a 30gtx with same exhaust housing, the a/r 71 or maybe use a smaller house... Depends on how you want your car to run. Road use? track? gravel? etc.... Both sizes 30 and 35 can be very equal depending on the exhaust housing. I would have chosen the 35 with a smaller exhaust housing