Stock for stock its no contest. However, you could get the Z and use the spare money to modify it... That would be a lot of fun if you like that kind of thing..
If you like to buy fast factory cars then get the GT-R. A Z06 would be fun too.
One thing I can tell you is that the GT-R won't be a bore like the RS4. The GT-R is very rear wheel biased so you can slide it around quite a bit too.
But a monster 370Z would be awesome too...TT with a cosworth block and a Sequential tranny. Moton suspension...
....I gotta go..