I like 18's in back,tire choice is growing.
Another site I visit got into a talk got into a talk about tire availability. After sorting through the offerings on Tire Rack and looking the available sizes from 255 to 335, and taking out the competition and snow tires, it came out to something like 5 in 15", 10 in 16" 100 in 17", 125 in 18", and 75 in 19". I wouldn't say 17" sizes are going way, but whatever diameter the OEMS are playing with are going to have the biggest selection. Regardless of size you choose, also check out weights as well as appearence. If you are putting a wheel under your car that weighs more than the stock rallye, you will notice the change in accelleration and braking capabilites. Because of the size and design, some aluminum wheels do weigh more than a steel 15" wheel.