You haven't really described the terrain, weather, or destination enough to really be able to say much. The kind of bike you will want depends a lot on how far you're going, how hilly it is, how hot and cold it gets, availability of alternate transportation (for those sub -20 days), your physical condition, how much stuff you need to carry, availability of a shower at your destination, dress code at your destination, and storage at your destination.

There are five or six bicycle commuters at my office. Four of us regularly ride the same route of around 12 miles. We're riding a Steelman Eurocross (with new rack and a backpack on lots of gear days), a Specialized Sirrus (with rack), a Surly Crosscheck (with rack), and a Specialized Allez (with backpack). For summer, we're all running on a slick. The Eurocross and Allez have 23x700 Armadillos, the other two bikes have similar tires. I switch to cyclocross tires on the Eurocross for winter commuting but will probably try out Nokian studden tires for the icy part of late winter/early spring.

I'm hoping to get a trailer next month to pull two weeks of supplies with and then commute light for the rest of the two weeks.