I've use Trial Mountain to tune my cars and at 500pp, the GTR is two-tenths quicker than the CSL. At 550 I still need to run the GTR, but the CSL is currently my fastest at 550pp (faster than the NSX-R, Ford GT, 458 Italia, etc).
On a subjective note: based upon my tune for each vehicle, the CSL is easier to drive balls to the walls without the rear end sliding out. The GTR is a little more tail happy when I drive it 100% but when everything clicks, the GTR is slightly faster. I tend to use the CSL more often since I can run laps more consistently than the GTR.