My tyres are 215x35x18 so fairly low profile, with the 18's alone the ride changed a bit and was a bit harder but wasnt bad by any means. I liked the ride better on smooth roads.
The main difference was when I lowered the car, the ride got rough but only on bad roads but i still think it is a better ride than stock, i dont know why though lol
The main thing is what you want, if you like the look of 18's then go for it.
Im glad I did and alot of people on here would say the same thing