Is the 3 series big enough for you? Have you found many times that it couldn't carry what you wanted, or just seemed to tight? If not, then for practical purposes, it's big enough, and any more is just carrying around extra metal and weight.
But if you really like the extra comfort of longer wheelbase for extended highway use, then go for the 5 series. But that's mostly for older people, and if you have no kids yet, then 3 series is best for you. Plus more in your price range.
Agree with your choice of RWD. It is more fun to drive and handles better, plus less complication.
If your concerned about winter, you can consider an LSD. Should cost less than the difference for X-drive or 5 series.