We've had an X5 (pre X drive), an X3, and a 325xi.

The 325xi was the best in the snow, but the X5 was fine. More ground clearance with the X5, but also more weight to deal with. I had the Michelin OEM all seasons on the X5, so that was the limiting factor. I had Michelin all seasons on the 325xi, but they were a different model Michelin, and better in the snow.

First important factor is tires. Real winter tires, not all seasons.
Second important factor is the driver, and his/her experience.
Third factor is the vehicle and the drive system.

As an example, I got the rwd 535 with Dunlop Wintersport M3 tires (on four wheels) up a very long hill that many AWD vehicles were stuck on, and which was then closed by the police due to poor conditions. The X3 struggled, but made it. The X5 couldn't make it. Tires over vehicle.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue