18's might give you better performance on a newly paved race track, but there are bumps on city streets... even on most skidpads. If you don't have any sidewall (18's and up) and have stiff suspension, then the tire might bounce off the pavement (while turning fast), instead of absorbing the bump. For that reason, I've seen many autocrossers who drive on the street with their factory 17's, but swap on 16's when they autocross.
How much would a 25" tall tire alter the final drive ratio vs a 24.4" tire? Guess if you have a 3.0 or turbo, you could go the route of tall tires instead of the 2.0 final gear.
So if you don't care about performance (cornering and/or drag racing) then I guess the motivation to purchase 18's are cosmetic based.... so do what you want. Just don't tell me I'll cut faster autocross times if I went from 17's to 18's.
