You will have more sidewall (a given with a smaller diameter wheel), size, and brand options in 18" wheels.
P.S. I'm going to try and fit 18x10 all around on my car in a couple weeks. Wish me luck.
m a g l i t o SGP engine destroyed itself after 4k miles. New engine dropped in 11/05/2006. Details coming soon. sig photo by me effects by ZXRider(SoCal) The build up begins - progress - reassembly part one - part two