Primary reason is that with 18" wheels I can drop the air pressure when driving in sand on the beach. Taller sidewalls allow for a much bigger "footprint" and more float in soft sand. Dropping the air pressure on 20" low profile tires does not improve sand traction much.
As for ride quality, I don't notice much ride quality difference with the 18" wheels. However, I went with 18" Michelin Defender LTX tires which have a deeper, more aggressive tread pattern so they are not as smooth. If you go with 18" wheels and Michelin Premier LTX tires you would have a much smoother ride.
