My 2 cents, Ride whatever you like. What you like almost always trumps everything else. With the short distances you're talking about nearly anything would work, even a backpack on a unicycle, wouldn't be my choice but heh.. I have a light touring bike, it's what I'd ride most likely. You have an ATB, I'll guess you'd probably be more comfortable more upright. European commuter bikes, Breezers and the like are made for your kind of situation. As distances increase the benifits of road/touring style bikes increases also. Where one makes that switch is personal.

From a practical standpoint, It might be argued that something upright will allow you more visibility. What's really important for everyday commuting is a rack, usable bag(s) and/or baskets, fenders and lights. And most basicly that the bike fits you. Is the area flat? you don't even need much in the way of gears. Got more hills, you probably want a few gears.