When I bought my Z I was also looking at the Corvette and Cayman S. I briefly looked at the 2010 Mustang but at that time they were still using the 4.6 and hadn't totally worked out their suspensions yet.
It came down to the Cayman S and the Z. I liked the build and handling of the Cayman better, but thought the Z won in looks and value. It was very close in everything else so I felt like it was the best buy and am glad I went with it.
