This question comes up a lot. In 2010, my housemate fractured two vertebrae. So, assuming she'd be sympathetic to people with complaints about heavy shoulder bags or backpacks, I asked her what she thought. (She carries a ~5 pound Macbook Pro in a briefcase, and usually is carrying a few other things in a second shoulder bag.)
She told me not to be a sissy.
You'll notice the difference in weight, but it's immaterial in terms of hauling your crap around. If you're drinking bottled water, that weighs more than the difference.