It's time for a new car... which will it be?

Still shopping around for a weekend car that will look a bit sexier than my old reliable (corolla).

I'm looking for a used model with under 60k miles on it. My budget is around 15,000 USD. Here's what I've narrowed it down to:

Audi TT (roadster or coupe)
Nissan 350z
Infiniti G35 (coupe)
BMW Z4
Honda S2000

I'm making the rounds test driving them, but haven't decided yet.
Which do you think is the right pick? Alternative suggestions are welcome as well.
I'll send pics of the car that I've chosen once I've bought it.
Thanks.