Hello Jollyranchers and welcome to the forum
I was inthe same position as you couple of month ago,really like both cars and had more than few test drives in both, hasseled all the local nissan and mazda dealers for two months.
Well I guess you know what i chosed in the end.
The RX8 it's a really good car, nice lines, more practical and not so thirsty on petrol ( on oil is ), but when I got to drive the Z it's was beefe, strong, and great great fun to drive!
If you want to have some fun, with the Z you just have to slighlty push the accellerator and you'll be while with the RX8 you would have to rev up over the 5k to set it in the sport mode.
Defenetly take both cars out for a frive and see which one makes you smile the most, your car, your soul.
Will this be you day to day car or just week-end?