I think the Z is the coolest looking one of the pack, but two seats and an outrageous price for the drop-top, and the fact that its back-end doesn't want to 'play nice' under hard driving (even though supposedly the live-axle rear of the mustang is 'stone age' in comparison to the Z!) really takes it off my list.
The STI is the best performing one of the pack, and I'd be (pleasantly) surprised if even a good suspension upgrade would make the mustang handle to the same level as the STI. The STI also has the technology market cornered- an excellent AWD and a phenomenal power/weight/displacement profile. But both the Limited edition and the standard edition just do not stir me... in fact the limited makes me yawn and the standard one looks a little rediculous. Cars and Houses should inspire and move you, as they are the purchases you will practically live in on a daily basis.
And for that reason, the GT Convertible Mustang really tops my list. With my employee perq, I can get a pretty much fully loaded GT covertible, mod the suspension and put in a short throw for about the price of the STI i'd settle for,or the Z I'd be happy with.
So it seems pretty much settled. Its only a question of saving up a down payment and finishing off paying off the credit card ;-)