Thank you all for your replies! In the last month since I posted, I ended up finding a '09 328i wagon (not xDrive) that really stole my heart (long-distance, no less!)...neither an AWD nor diesel, but I figured I could do the snow tire thing and it will be OK.
Right now, I'm currently awaiting for it's imminent arrival...the transport truck should arrive in an hour or so!
I really, really wished that the wagon got a bit better MPG, but theoretically it'll be better than AWD in that regard, so I'll try to get over it. Like krhodes1 said, it probably won't make much difference in terms of the cost of gas; I wanted to be as "green" as possible, but I will (and do!) make it up in other ways!
Thank you all for your advice!