If, as you say, you are serious about commuting, I would recommend a separate bike set up for commuting. I lucked out and got an older steel frame hardtail Mountain Bike - CHEAP!, and over the course of the last 3 years have outfitted it with narrower street tires (Michelin 1.75 wide Country Rocks), a rack, fenders, pump, and panniers. I used a backpack for a long time, and must retract any statements about backpacks being fine for commuting, at least for me. The rack and panniers are MUCH more comfortable and easier to use. Some days I ride one of the road bikes if I don't have any cargo, but I enjoy the commuter immensely too, in it's own way, and never worry about having to jump a curb, or take to the dirt if the truck coming up behind me looks like it wants my part of the lane(shoulder, what shoulder?), even with my 260# on board. I average 2mph slower on the commuter, that's the main difference.