The weight differences are interesting, because even though the numbers aren't large, they are significant.
I used to carry around a lot in my backpack on campus. I'd have my 13" Toshiba, a couple of books, and maybe a thermos in the side pocket. My back started hurting, though. When I asked around, it turns out that lots of fellow students and professors have back problems. Some of them are pretty debilitating. That's when I got a netbook and stopped carrying books. I can't say for sure that backpacks are to blame, but it surely doesn't help to carry all of that around on your back every day.
Of course, everyone is different, but one thing I would recommend is to get the smallest computer and the smallest backpack you can get away with. In my case, I was sorely tempted by the mba, but ended up with the mbp because it had been refreshed. I'm pleased with it, but it also means I have worked even harder to empty out my backpack and make it as light as possible. Now, I just have my mbp and ipad with some pencils and pens. In a few years I imagine I'll migrate down to an mba.
I should add that I am in extremely good shape. I work out every morning in the gym, swim a couple of kilometers, and do yoga at night. The back is a tricky thing, though, and I recommend you take good care of it.