[cscobra]

15" wheels were more esthetically pleasing and more original-looking to me, so I went with them. I also went with the Goodyear Eagle GT II tires (nearly all original Cobras came with Goodyear tires for reasons already well-known!), although I've read lately that certain Firestones may actually handle and ride better. Speaking of ride, you'll get a better ride with a taller sidewall (15" wheels) than with a shorter one (17" wheels). With regard to handling, you won't miss much in the way of handling with the 15" wheels unless you spend all of your time on the racetrack or continuously drive 10/10ths on the street. Tires for 15" wheels also tend to be less expensive.
Just my own thoughts and biases!