I compared messenger bags for months and found that in the end I wanted something good looking and functional, but most of all, light weight. It makes a huge difference when you carry books or a laptop everyday.
I would recommend against leather or even canvas if weight is a concern. I eventually went with a light nylon bag from coach and it has held up perfectly for almost 5 years now. It cost $300, but worth every penny.
Here is one very similar to what I bought:
http://www.sonsofstitches.com/buy/me...essenger/30323