If you live anywhere where there is ice and snow during winter, go with the 20" wheels. 18's are way too tight. I have a car with winter wheels that are a size smaller than stock and there is not much room, which causes them to be frozen up on cold mornings. Real pain. I have 275/60r20's that are slightly taller than stock...give me a taller sidewall with same width.
We get snow and slush, if there's anything left in the wheels when the car is parked overnight, it freezes. Only issue with the stock wheels, is that it causes a misbalance and a speed wobble, but when the rotors and calipers are too close to the wheel they get stuck. Also with the salt and sand used in the roads here, it wears out the rotors really quick. More space there is between the calipers and wheel there is for the winter, the better. If you don't live anywhere cold and snowy, no issue.