I think sheepskin seatcovers are the best of all. Never burning in the summer, nice and cozy in the winter. The first thing I do when I get a new car is get sheepskin seatcovers. My butt's worth it.
I'd concur with that. My Dad's car has sheepskin covers in it, and I'll admit they're pretty nice in hot weather.
My mustang has [black] leather seats, and they do get pretty toasty if I leave the top down in the sun. BUT: the instant you sit on them, they cool right down. Its not like vinyl that sears onto your skin and sticks there.