What's a good backpack or messenger bag for a computer?

I want a new backpack or messenger bag for school this fall. I’m going to be carrying a 13” MacBook and some books/papers. I plan on upgrading to a 15” MacBook Pro next fall and would like to use the same bag if possible.

I see the http://www.timbuk2.com/tb2/products/laptop/laptop-messenger-style/laptop-messenger#product=2-78703 and think the medium size will work best, opinions?