Haven't driven a Cayman S, but every comparison I've seen rates the Cayman as better handling and worth the price. If you found it's transmission inadequate I'm not sure you'd be happy with the 7AT either, though it is an incredible automatic.
If you're worried about needing back seats, you are looking at the wrong car - get a BMW. No one can say if two seats are adequate for you, but they are for every 370Z owner
