John, there's nothing wrong with the Sport or the 18 inch wheels. Its all personal choice. The only thing that bothers me personally about the Sport is the lack of alloy design. You only get one design choice albeit in three different colours. I saw no reason to buy the GT over the Sport when placing my order for the second RCZ. The Sport has everything you need and a less harsh ride due to the 18 inch wheels.
I've specced my second RCZ with everything that I wanted and it's still cheaper than the GT and when I get the seats retrimmed, I have a "one off" car, which is personal to me. The Sport model also retains more of it's value than the GT, albeit a small amount, but still more.
Don't let the GT owners talk you out of the Sport model, it's a great car

. With the electric seats, if you want to move them forward they take an eternity to move, not good when it's throwing it down with rain. The manual seats move in a jiffy