Timbuk2 Classic Messenger - What Size for MBP 15" with Sleeve?

I have a Waterfield Designs neoprene sleeve on my MBP 15". I'm looking to upgrade to a Timbuk2 messenger bag now that I've started grad school. Anyone know if the Medium bag is big enough to comfortably fit a sleeved MBP, books, gym clothes, etc.? Or do I need to look at the Large bag? The colors and prices on Mediums are pretty enticing.

Message was edited by: Emmett Lyman