Youve got two things to consider, unsprung weight and rolling radius.
Increasing a rolling radius will make a car accelerate more slowly but will increase top speed, its effectively the same as making the gearing longer - an inch isnt a lot but it will make a difference, the car will feel more spritely on 18's than 19's.
Unsprung weight is the same as normal weight, but reducing it has considerably more effect on the cars dynamics - reducing inertia means that everything goes better. Ive played with various different wheels on my driftcar and the difference is very noticable, even between an 18 inch 350Z Ray and a 17 inch SSR Type C (featherweight rims), everything from the turning to the suspension just works better.
Youre always going to be best off with the smallest, lightest wheels that will cover your brakes, but they are rarely the ones that look best