I'm in the market for a new cycling backpack for commuting to and from work preferably a mid size one.

There are so many great cycling backpacks out there and I don't know which one I should get other that It's needs to fit 15" laptop and be very water resistant/waterproof.

The backpack that I'm currently using is the timbuk2 especial tres which I really love however it's too big for me most days so I want something smaller.

I want to get your help and see if you could recommend a backpack that would suit my needs?