I was curious as well. I bought both but the ZR-1 needs a chassis rebuild so I can't say one is better than the other as far as handling. The ultimate test would be to set both cars at the same PP and tire choice and set them loose at a track.
I found the Z06 RM to be better on default settings simply because it had some camber tuned in.
Ofcourse tuning the ZR1's suspension could make it hand as well or better than Z06. The chasis should be the same when in RM trim (ZR1 having got some of the racing DNA of Z06 based GT racers) so the only difference i can think is the weight and performance of the engine...