Messenger bag. Backpacks are too much of a PITA to use.
But, on the other hand, my fully-loaded laptop bag, with JUST the laptop and associated equipment (charger, inverter, external HDD, spare batt, mouse, mousepad, TI-89, multimeter, ethernet cable, USB cable kit, Uplink codesheet, reimage disc set, GPS reciever, cheapie WAP/switch, blank CD-R's, etc. etc. etc. etc. etc.) weighs like 40lbs, and that's a LOT for one shoulder.
Also, I can't cram any books in there, so I have a seperate backpack for those (while I still have them, suspect that soon as I'm out of HS, I won't be using paper much at all, and I can definitely find room for a coupla textbooks). It's uncomfortable (and undoubtedly looks fairly hillarious) to carry both at once.
I guess I should mention that my laptop is primarily a mobile desktop and needs to be used for severe-duty work in a number of different environments. Cars, customer calls, school, basically anywhere except at home.