If your commute is around 5 miles each way, save your $ and get a pair of slick tires for you mountain bike, and outfit it with a front/rear rack and any other accessories you might want. Unless you had a 10+ mile commute each way or anything else longer, your mountain bike will be more than sufficient. By some good bikes clothes and accessories rather than an expensive bike for a short commute. There would not be more than probably a 1 minute difference between your mountain bike and any of the others you are looking at on your ride.