I have the 18" HFP for summertime, and the 17" stock wheels for winter. Overall, I would say the difference is very minimal as far as ride quality/comfort between the 2. Only thing I notice is on the 18" I have the Pirelli tires, and they are rough until you get them warmed up, and then very smooth and sticky.
Other thing to keep in mind is price of tires. 18" tires are more expensive overall.