They're both great cars and both have unique attributes which make them a joy to drive.
I had 2 192 RX8s before I brought my Z. The Z has more power and it's a bit better on fuel so it's a no a brainier Or at least I thought. To be honest, the Z didn't feel that much faster although it is, it's power is more linear so it can be deceiving. Not one of my passengers ever said it 'felt' faster than my RX8 (it was). It's a 2nd car so the fuel wasn't an issue. After about a year I found myself getting bored. I sold my Z and was going to get a 370 but then after I test drove one it didn't feel much faster than my 350 @ 289bhp. I think you need to go FI to get that 'oh sh1t' feeling when you put your foot down (can't afford that to do that to a 370).
Then I test drove an R3 RX8, although it doesn't feel as fast, it handles like it's on rails and revs to 9.5k like it loves every second of it. The gearbox is so tight and precise. I just got more joy driving it than any of my other cars inc the Z and 2 192s RX8s. It's something you need to experience before you say, 'oh they drink fuel, oil or they need rebuilding every 40000 miles' (not true). I ended up going back to the RX8 and getting the R3 model this time. If I had the money, I'd have both cars.
Not my car (below) but the same apart from LHD.