Most will probably say the 18s are esthetically more pleasing than the 17s....but you have to be careful and watch the weight of these versus the 17s. Those cheaper, lower end chromed 18s can be extremely heavy, reducing performance. Add in the fact most 18" tires are also heavier than 17s, an you can add significant weight. If you're going to go 18s, you probably want to stick with the upper end manufacturers like TF and CCW that make very light products for their size
This--and to make matters worse, it is unsprung weight you are potentially adding.