Thanks for the response. I think I'll probably spring for the medium. I actually have been using a backpack for the past few years (Timbuk2 Hacker) which has for the most part been awesome and super-duper tough against the rain and tarmac here. In fact, I've been resistant to the idea of using messenger pack for a while since I really like the security and comfort of backpacks. The only issue I have with it is that its really a huge pain to get stuff in and out of it while on the go (in particular, getting out/packing up my u-lock or bike lights between classes, or dropping off mail pieces at mailboxes). Also, the compartments in the backpack make carrying laptops and papers a cinch, but leads to compromises when moving larger things like cereal boxes/jugs of milk/cabbages/etc.

I know walking in msnger bags generally doesn't work, but I'm usually packed very light when trekking in the city.