I ordered my x6m with the 21" option and just changed for 20" winter set last week.
Having driven on both sizes here are some of my thoughts:
- The original tires are useless bellow +5 degrees Celsius so even if you are not required to change to winter tires by law you should really consider it
- I personally love the look of 21" and am even looking for some 22" options for next summer, however the ride is considerably more comfortable on the 20"
- Handling wise I don't feel much difference between the 20" and the 21" as the car is just as planted as always
- If you personally don't see much difference in the looks or just don't care about it that much then I think you should go for the 20" as the car will be softer. Then again depends on the roads in your area. I live in a country with very bad roads so I really appreciate the added comfort of the 20" winter set.
IMO I got a really nice winter set, and the comfort improvement is good, however, having had them for a week I already can't wait for the summer to be able to return to the 21" as I think that 20" just looks to small for such a big car
It all comes down to which one do you prefer - looks or comfort. The tradeoffs are quite apparent and noticeable so good luck in your choice