I will chime in as well.
18's for track and recommend square setup. If you don't track and just want to have fun on city streets go 19's staggered. Looks way better and you will barely notice the difference in ride. They also have a little more rubber on the road but they are siginificantly heavier. I have both. 19s for DD and 18s for the track. Perfect combo in my opinion.
Good Luck with the purchase.