Before this year, I would have said Cayman every time. Corvette's just don't interest me. But the new one is stunning, and delivers awesome performance. So it's a much harder choice.
I'd still lean towards the Porsche, though. More fun to drive a slow(er) car fast, and all that. Plus, even though the 'Vette has come a long way in the looks department, the Cayman still wins that contest.