I have done the pannier setup and it is nice. Mine had a strap that pretty much turned them into a messenger bag. That was back when I had books to carry but for the last two years it has just been cloths so I use an Osprey Talon 11. Great bag! this one has over 15K miles on it and still works like new. I should water proof it but were I live it does not rain much.

Commuting 30 miles a day I would say backpack over currier bag any day. Seriously that thing swinging around like that, I tried a few and they always rubbed my shoulder. If you need to carry more than just light stuff go for a pannier setup.