Personally, I love the look of the 20's on the X5 in summer(wife hates the look of the winters). I have 211's as my winter setup and i have to say they are incredible. Yes they don't look as good but I can say that they perform above and beyond what you may expect. The Square setup lets you rotate the tires which is a bit plus. The Xdrive does not wear the tires the same and my rears(2007, 2011,2012) all wear/wore the back faster than the fronts. Since this is the wife's daily driver, she has driven in every crappy, icy, "don't drive" conditions with not one concern.

The 19's are a touch softer than the 20's. You can still push the X5 to about 85% with the 255's but in the winter, do you really need to push it that hard. If you have 20's, DO NOT run 18's. I made that mistake and had 255/18's originally on the 2007 and I got rid of them in close to 1 month. It was so soft I moved to 19's yes less side wall(bad for pot holes) but in Chicago, i have never had an issue.

__________________
2012 X5 50i ZAP(2VA, 336, 465), ZTP, ZPP, ZPS(6VC), ZCW, ZRC, 322, 4UB, 4NC, 610
2011 ///M3 Silverstone II
2011 X5 50i(lemon)
-- SOLD --
08 550 Msport
07 X5 4.8I sport
04 545i sport
02 4.4i sport
01 540I M/sport
00 SL500
97 SL500