Roadsters are purpose-made for riding on pavement. As the old saying goes, "if you want to go fast and far, get a roadster". Doesn't sound like you are particularly in need of speed or distance at this point for your 20 minute commute, so it's probably not too important. A well-set-up roadster is quite comfortable, but many folks are put off by what they see as an uncomfortable riding position. They don't seem to realize that you're not supposed to ride with your hands "in the drops" all the time.
Mountain bikes make great commuters. They are durable, unlikely to flat, and can easily be fitted with racks and such for your books and gear. The components are rugged and will stand up to much neglect and abuse.
One thing you may consider (I'm in "campus" law enforcement) is that MTBs are far more likely to be stolen. I can't recall taking a theft report on a roadster in the last 6-7 years. We get mountain bikes stolen constantly. At my school, with maybe several thousand bikes present during the school year, I'd say that 95 percent are mountain bikes.