I chose the 15's my 'rex is currently wearing due mostly to my shallow wallet. A guy at a local speedshop had a set of GSR wheels that he sold me for $250 with tires. They're only 6" wide so they're not that much better than stock (14" by 5") but they look nicer and allow for better tires. I've seen CSP prepared 'rex's pull over 1.1g's with 15" wheels, they were probably wider than mine but hey, i'm cheap! Going to larger wheels with a larger overall diameter than stock will also change your gear ratios, if you drive a 'rex than go
HERE to see exactly what difference there is.