I own a 370z and have owned an RX-8.
The RX-8 has ok power on the top end due to the high-reving rotary engine, however it doesn't have the torque and low-end power the Z has. It also is setup for passengers which I never had more than 1 of. Handling-wise it does pretty well but I prefer to have more power behind the car to get it through the turns and such smoothly.
The 370z is pretty raw. It can jump off the line quite easily, has pretty good power until you hit towards 5500-6000 then it sort of goes limp. Handling-wise it does well in corners and straights however VDC is pretty amazing in corners even if it makes your car a bit under-powered. Turn it off and your car will take corners a bit faster but you better know how to drive
In the end it's all about what you want and what makes you happy. I obviously went with a Z and am happy.