I have leather seats in my car and I live in Florida
. I dont even experience this "way too hot" seat thing people seem to complain about. Sure my car windows are tinted, but I would assume most poeple who are worried about car temperature would have their car tinted as well.
Between leather seats and a spoiler - I would go spoiler. If your back seats arent leather as well I think it would just look out of place to have half the seats in your car leather and the other half not. That and I love the GT500 spoiler and it will probably be my next mod as well.
Why installed by the dealer though? You could save yourself $300 and install it your self. From what Ive read its not that hard - seats or spoiler.