I am running 19's and if i had to do it over again. i probably would have gotten 18's. 19's look better but i think perform a little worse because you have to watch out for road conditions. I guess it depends on where you live but i live in los angeles and the roads are shit. Running on 30 series tires, going over potholes, hoping not to bend any rims getting stressful after awhile. But if your more into looks then go with 19's, but again thats based on pure opinion.
better for what? for looks the 18 if that is what you like i don't think there is a measurable increase in grip for average sport car driving or drag racing over the 17" . unsprung weight goes up in a larger tire wheel and may actually worsen performance .