I just went through this on my X5M. I felt the stock 20" staggered wheels probably weren't going to cut it for the long Montana winter so I bought a set of 9x19" 298M wheels. For me the tire choice was between Blizzaks and Pirellis. I chose the new Pirelli Winters over their decade old Ice and Snow design primarily from the recommendations of the Cayenne crowd. The Pirelli Scorpion Winter was just introduced this fall and is to replace several of the Pirelli winter product lines. My size, 255/50x19, is available as both runflat and non-runflat.
I can't tell you how they are yet as the truck won't be going to Montana for 3 more weeks