I voted for the STi. You get a quick car in the corners and straight line, and a warranty to boot.
As for the 300Z, I dont see the hype. Its a butt fucking ugly car IMO, and mostly ALL of them are slow as bricks. I ran one last summer with upgraded turbos, full exhaust, and a fresh motor and he pulled a half car to 120mph. GHEY!
Just FWIW, the MKIV Supra would be up there with the STi for my choice.