It is my firm belief that you will enjoy the Supra more. I wasn't very impressed with the 350Z I had a ride in. The "pull" it gave was annoying, since the owner had his seats leaned back, thus straining the neck a bit. Maybe it would have been better with some upright seats instead of gangsta-lean ones, but the pull still wasn't as hard as I had expected it to be.
Be patient enough to wait for a used 6-speed MkIV in great condition for between $20-30k
Spend $1500 to BPU it.
You aren't going to want a Z if mods are what you are after. You will have to wait for the warranty to be up, and by that time, you might be sick of the car's lackluster performance.
Oh, and if you DO buy a Z, buy the stock model; don't get the S or R-tune. Spending an extra $5-10k for 10-20hp from the factory seems a waste, when you can pull a ghetto BPU [open downpipe, swiss-cheesed airbox, bleeder and a $75 Greddy BCC] to gain an extra 100hp from the MkIV. Hell, a proper BPU with the "best" components will only cost $2500, and that's with a HKS Ti Racing exhaust, a high dollar electronic boost controller, a Blitz filter and a Greddy BCC, with cash to spare for some cam gears to squeeze a lil more out.