GTO4Life, just test-drive both, you'll see what I mean. I know the RX-8 isn't as fast as the 350Z, but the thing can REALLY handle, it's super fun to drive, and it has the practicality of four doors. And yeah, I felt that the RX-8 was more comfortable and funner to drive. Sometimes, there's more to a sports car than raw power.
Looks is an issue too. I personally think the 350Z looks horrible compared to the Z cars of old. The RX-8 is very sharp looking. Even though the extra set of doors kinda ruins the sports car image, they make the car a lot easier to live with.