I currently have both. The Zed is a lot better in nearly every respect apart from practicality.
RX8 - Handles like it's on rails (in the dry), nice interior, well equipped BUT appalling fuel consumption (I've averaged 14mpg in a year), unreliable engine, problems with rust, engine easily flooded, sounds like a hairdryer, no torque and is driven mostly by women. I really regret buying it now, as it's proving impossible to sell as well.
350Z- Looks fantastic, sounds great, well equipped in GT spec, gets compliments everywhere you go, better on fuel than RX8, uses less oil and faster, BUT interior is low-rent, clutch is a weakpoint, is massively impractical and that's about it.
I should have bought the Nissan in the first place or as the saps on the RX8oc say: "A fat Datsun".