Okay, here we go. I found some times on the same track for both cars. Now these are the best credible times that I could find for both cars. There is one detail im missing about it though. I'm not entirely sure if they were compared on the same day (then again they may well be), but its still on the same track, from the same sponsor and driver. Please this may not be entirly accurate (given of what I said), but its the best I could find.
Road and Track did a article called "Sibling Rivalry", which compares a multitude of cars. They compare 2 cars of each company with a total of 6 companies. The Z06 and the Modena both have recorded track times for this coarse. Both of these cars ran the fastest two laps out of all the cars (including a 911 turbo). The Z06 ran a time of (2:08.39 seconds), the Modena (2:07.65 seconds) on the Thunderhill Park coarse (California). Very close as I said, considering this was a 01' Z06. Would the times of been closer with an 02'+ Z06? I don't know, but maybe. Given that I don't have enough information to say if these were recorded on the same day, they may not be the best times you can find.
These two cars are very very good on the track, no doubt about it. What would the main factor be between these two in a matchup? If you ask me, now I'm going to say the driver, no question. Their both to close to call accurately.
If you would like a link i'll glady post it, just let me know.