A Spyder, of course...
Well, I own one, but it's an easy decision. Living in California, I drive with the top down whenever it is more that 70 degrees or so and dry, which is a lot of the time. Also, unless you are trying to set bleeding edge track times, I have to say I love my Spyder on the track. I've taken it to Laguna Seca twice, Sears Point twice, and Thunderhill once (unfortunately in the rain, so the only time I had to leave the top up!). Great fun.
I also think the Spyder with the top down is a much better looking vehicle.
The only reason I can imagine to get a coupe is for the back seat, if you need it.
But, as I said at the beginning, I have a Spyder.
Mike