I am generally a backpack fan, because I think they're more versatile, generally, and they make the transition to the train easier than a pannier would (I think -- I'm basing this on years of motorcycle saddlebags -- I've never used bicycle panniers). However, the weight you're talking about probably exceeds my comfort zone for cycling with a backpack.

While a well fitted and packed backpack is very stable, and works more like an extension of your 'self', I would actually start to get nervous about the weight altering your own balance if you're talking about a full size laptop plus two major texts (I'm imagining my largest law school or science texts, here). I carry a lunch, a change of clothes, one or two casual reading books, a spare water bottle, minimal bike parts (tubes, patch kit, lights, etc...) MP3 players, PDA, phone charger, etc. -- heavy load is probably 20 lbs with backpack itself. Once you get much over 30 lbs, though, I think balance would be an issue (depending on your own size, of course... the percentage of your own weight that you're carrying matters).

I don't know that the sweaty back is the huge deal people make of it; if it is hot, you're going to sweat anyway, and I don't think the addition of the backpack makes a big difference (to me). Sure, the backpack holds more of it and on your back, but for a 6 mile ride, this is a minor inconvenience at worst. Perhaps if I were riding more than an hour I'd feel differently.