Ride comfort mainly depends on the stiffness of the suspension and the ratio of the tyres. Any car with 20" rims will almost certainly have "sports suspension and ultra low profile tyres. If you go over a 10p piece you'll be able to tell if it's heads or tails!!!!. If you decide to go for 18" rims the tyre ratio will probably increase by two sizes, but because you will still have the stiffer suspension you'll probably find the tyre wear will deteriorate. This will increase the ride comfort but don't forget to advise your insurance company who may hike up your premium. I would suggest you look for a car with the smaller rims that will probably have softer suspension.