I have repeated this a few times... and this is just to give you another option. I went from 19s to 20s (aftermarket)... the reason your dealer said to avoid those 20" upgrades is because they are summer tires. You didn't mention if you have a 3.0 or 4.8 but I'm guessing the latter since the upgrade option was available to you at the dealer (not available for the 3.0 in the books). He was looking out for your interests because of the snow and I agree with him. If I had the option, I wouldn't want the summer-only tires, but would take the 214s of course - lots of us have gone aftermarket.

If you're going to push the vehicle all the time, you may have valid concerns with the larger wheels, but day-to-day driving, going from 19s to 20s, you don't notice much of a difference. The other thing I preach is this... unless you're driving around in snow ALL THE TIME, get all-seasons and stop worrying about changing tires twice a year or even having to store the extra set. I have now survived 2 winters and driven in a good handful of snowstorms with the staggered 20s on Toyo Proxes all-seasons with zero problems. I sold my 19s to someone who had the summer-only 20s, and I haven't looked back.

If you're going for aesthetics, do everything you can to stick with 20s year-round. 18s or 19s may be effective but they do absolutely no justice to the X5. All-seasons are not bad unless you're using the X5 as a race car (pushing its limits) or snowmobile (same thing). The summer tires bald easy as well... for obvious reasons. The all-seasons are FINE for day-to-day including snow here and there - it looks like you are familiar with all-season performance already anyway. I stare at the M3 across the street that has to stay home on days like yesterday (and days to come) while I drive my X5 everyday - it's funny, they didn't even bother clearing the snow off yet.

BTW, brand new from the dealer I think 214 summers run near $3400, just to give you an idea. If you're asking about value, I wouldn't pay $2700 for used, summer-only tires (even if that's what they go for). Again, just to give you another option, you can do replica 214s and all-seasons for $2k brand new, but I assume you'll need RFTs as you have 3rd row. Oh and skip the running boards - it's hard enough getting out as it is without the boards if you're shorter. My 2c.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr