ZR1 all the way. That car would be so much more fun than the GTR. Of course, I agree that they arent in the same league, more like Z06 vs GTR. GTR may still be slightly lower on some things, but atleast they are more comparable.

GTR vs ZR1
http://www.youtube.com/watch?v=mC-PQca6FJU
ZR1's GTR destroying official run at the ring. Scroll down in the comments and you'll see where it stands for 08's times (2nd best). I love how the guy seems to have a hard time with it in some places. I bet its a beast on the track.
http://www.youtube.com/watch?v=k6mEirkQN8o