With Yoda, agree I must. Ideally it would be cool to have a sleeve (maybe an STM neoprene one) to protect the laptop itself and a bag which would serve as both a laptop bag and general purpose bag. For my old 12" Powerbook, I had one of the old design 12" Crumpler McBain's Baby sleeves (which had excellent padding) and put it into a Crumpler WonderWeenie with all my books/files/other stuff.
If you can afford it, get both for that extra bit of protection and versatility. If not, then I'm sure there are heaps of great bags with adequate laptop pockets as well... good luck!
I've been using a Marware sportfolio , and the nice thing about it is that it's very light and perfect to carry a laptop + a few other things, and small enough that I can stick it in a backpack when I fly and need to reduce my number of carry-on items.
Get the sleeve unless you want to look like a huge nerd. Laptop backpacks are way bigger and you rarely see anyone on a college campus with them. If however, looking like a nerd is up your alley, than I would get the backpack because you will fit right in.
I highly recommend a Tom Bihn ID Messanger bag... The quality is top notch... Only problem is that if you want one for fall semester you'd have to order it now.. Mr Bihn seems to have quite a wait list these days
I swear by my Brenthaven Professional backpack. Built like a tank, lots of room, plenty of pockets, a removable sleeve, and they are EXTREMELY well padded. It's almost as good as Applecare.
I have a Powersleeve from http://www.booqbags.com/ . They make really top quality stuff, I've been very happy with my purchase. They also have bags if thats your sort of thing. I prefer the sleeve because I can just put it in my backpack and carry it along with books and notebooks or whatever else I need to carry around.