A mate of mine has an RX8 and he feels exactly the same as you do. Although on paper there isn't a lot of difference between the RX8 and 350z, the way the power is delivered is completely different. Every time he gets in my car he always comments on how much more powerful the 350z feels and how you have to rev the nuts off the RX8 to get decent power levels.
If you can sacrifice the back seats then I'd say go for a 350z. Take a test drive, what do you have to lose!