I use a Camelbak Ares with the bladder removed (about $50 at the usual online discounters) - it holds my laptop + a few papers, change of clothes, raingear, arm/leg warmers, lock & tools/tubes (but not much room left over).
Other backpacks may have similar features so you may want to look around, but what I like is that it has a very slight flexible internal frame & ventilated stand-offs which help keep you fairly cool. The internal frame is nice - I never have anything poking through & never worry about "torquing" my laptop. It has a beltline strap which keeps it tight to the body.
Opinion time... I have plenty of experience with panniers & some with messenger bags & I way prefer the backpack for my commute. I ride hard enough that I get sweaty no matter how I carry my stuff - I change clothes at work so I don't mind a few sweat spots on my back. I like the stand-offs & the better shoulder support vs the messenger bag. A messenger bag has some advantage if you are loading/unloading frequently but will also leave (usually large) sweaty spots. Good backpacks & messenger bags feel like part of your body so they don't significantly impact your bike handling.
Panniers are good if you have back pain issues or are carrying large loads or for extended rides (ie - weight x time = sore back). My commute is only 40 minutes each way & I don't like having my bike handle like a dump truck. If you are commuting in your work clothes & ride slow (not sweaty) then panniers might also be the way to go.