I just put on the winter wheels as well - 18" base MDX wheels with Michelin Latitude X-Ice Xi2 tires. I think I like the look of the 18" wheels better than the 20's - the proportions seem better to me.
I always found the 20" to look weird in the MDX.... Also the car has the height, AWD System and Suspension to go OFF ROADING but if you put a 20" there its OVER, 20" cant do nothing on sand and they got very little sidewall for slight mud or anything light duty off road. I think 19" would be my Top size for the MDX to keep the proportions and usability.
18" for maximum utility in winter or off roading
19" for slight show-off and keeping the proportions even
20" for absolute ON ROAD performance, zero off roading or winter conditions.
21" for why?
22" You have the wrong car
+23" Now you are just trolling