But if I'm on a trip and out dining with a bag then the backpack annoys me. If I'm sitting I want my bag stable at my feet or on an extra chair. A backpack never sits perfectly like a Co-Pilot type bag would. I'm always fussing with the bag to make sure no one trips on it or it doesn't fall.
For your situation it depends on how often you'd have to load/unload the backpack. I don't take a bus or train so I'm only guessing but I can't imagine having to load/unload many times at all when commuting to a destination. If my walks to and from the mass transit location were even moderately long then I'd go for a backpack very quickly over the briefcase style. But that's me.