you wont bend or damage an 18" wheel because of potholes. there is far too much rubber to do that unless you are doing rockhopping or something. keeping your 20" original wheels and having a designated 18" winter setup will keep your summer wheels in mint shape and give you a softer and smoother ride on the 18" during the winter months.

i decided to keep my stock 20" 214 wheels as winters and installed 275/315 toyo designated winter tires and purchased 22" summer wheels with all seasons.