When you add up the pros and cons of 20s vs 18s, the 20s don't make any sense unless you must have them for their looks. 18s are cheaper, ride better, wear more evenly, produce less bump steer, are less prone to sidewall damage on crook roads, protect your wheels better, and impart less impact loads into your suspension. Higher fuel use with the 20s as well due to the extra aero drag and increased rotational inertia. Of course the roads you drive on play a big part in this and 20s will, of course, be a much better proposition on smooth roads then on rough roads.