Are the roads/paths generally clear? Twenty miles is going to be a long trip in the winter if the roads aren't in good shape. I would get a bike that could take the 35mm Schwalbe Marathon Winter tire. These tires have lots of studs so they're good on any icy patches you may encounter. They also roll pretty well for a studded tire.

Their downside is that they're not so great in snow over an inch or two. Given the length of your commute, riding in more than a couple of inches of snow may not be the best thing anyway. I'd be tempted to drive or take public transit on those days.