Rack. Well, with conditions.

There is no single 'ideal' commuter bag. Some days, I don't need anything. Others, I'm taking a half load of laundry to and/or from work. Still others, I'm hauling a couple weeks' worth of lunches to the office. Or I'm picking stuff up on the way home. Maybe I've got leg and arm warmers, a vest, and maybe a long-sleeve jersey in addition to everything else.

I go from riding with just the stuff in my jersey (wallet, phone, etc) to a stuffed handlebar bag, to a rack with a few things on top, to one or two full panniers.

The ability to adjust to meet that day's needs is what is valuable.