There is a very limited number of options for replacement tires in 21"(and 19" to a smaller degree), so if you plan on keeping the truck for a long time the 20" wheels will be "easier" to live with.
Doc: I've got a Q7 Prestige ordered and in-bound and was going through the process of sourcing some winter rims and tires (I live in Minneapolis and travel extensively in the Northern Midwest in sales). As you know my Q7 will come with 20's however based on everything I've read and experienced in the past with my current vehicle (BMW X5 35d), downsizing slightly, in this case to 19's seem like they'd offer better performance characteristics. Would you agree? However based on what you stated above, it sounds like I'll have a more difficult time finding tires in 19's vs. 20's, although perhaps you didn't mean with dedicated snow tires. Ok, hijack over!