I used a 13" MacBook when I was in school and I looked into replacing it with an ipad but didn't end up getting one.
I carried my MacBook everywhere and it was smaller than most of my books, so size was not an issue. In a few years when more textbooks are e versions it may be a good investment, but for now, I'd stick with a 13" size laptop and get a tight fitting neoprene case (I had an incase) and just toss it in your bag.
Plus a laptop is much more useful when you're done with school. I use mine everyday and it pretty much lives on the coffee table. I cannot imagine surfing all night on just an ipad.
-Sent from my MacBook using its continuing awesomeness.