If you can, try bike combinations with the bag outfit you want to use for commuting. The fit and positioning and balance of the bags on the bike can make a big difference on a daily commute. I ride 13 miles each way on an old "stiffy" mtb with slick tires. Kind of like what Andy_K described, so a mtb leaning towards a hybrid. It has good gears for the two moderate hills I take every trip but the flat bars are limiting in traffic. I have pleanty of road bikes, so sharing duties on one bike is no concern, but I do like the bigger mtb wheels and tires for the rough roads and baggage weight. Try some different combos and judge for yourself.

Jim