I daily drive my boxster, and I used to daily drive my 2006 325xi sedan.
The only problem I've had with the boxster in bad weather is when the RWD gave problems going up a hill when it was covered in snow. Likewise, a FWD car would give problems as well, just in different situations.
AWD is really the best for the snow, but I'm sure in canada they have snow plows ready by 4 am.
I was, however, fine on slushy roads.
The 325xi was a great car. Zippy, good handler and looked pretty good. It's more practical in the sense you can seat other people, but the boxster is much better IMO.
It has a good amount of trunk space for the type of car that it is because it has one in the front as well as the rear.
The advantages are everything you'd think they'd be: Sports car fun as opposed to sports sedan fun.
So basically it comes down to what you want. Do you need the more practical AWD sedan as much as you want a sports car?
I'd say take both if you could