Go with the Sport Suspension. I don't own an X5, but I have been in all the X5's (except the 4.6is) with and without the Sport Package. With the Sport Package, the ride is noticeably stiffer, but it's not bad. Up front, the ride is still smooth and comfortable...it's the back seat where the ride is really harsh and bumpy with the Sport Package.
If I were you, I would definitely go with the Sport Package. The X5 3.0 standard wheels look a bit clumsy, and if you have ever been behind one on the road for a while, you will notice that the car sits kind of weird. The X5 3.0 Sport Package wheels look so great, and it improves the handling at the cost of a bit of a stiffer ride, but I think it is really worth it. As for all-weather drivability, you shouldn't have to worry because the X5 3.0 Sport Package's 18" tires are all-seasons, which should do fine in the snow. There are a lot of people here in NJ who have the X5 3.0i with the Sport Package and they get by fine in our winters with the Sport Package's 18" all-seasons.