+1000 on the hard luggage. I commute 30 minutes one way daily on my bike. I wore a backpack for a long time until I finally got smart and bought hard luggage. I don't know why I didn't do that WAY sooner.
If you have your heart set on spending money on a special backpack for bike commuting, get a cheap halfway decent Jansport or similar backpack. No need to spend a fortune on a backpack. You don't need a special motorcycle one. If, however, you are really deadset on spending a lot of money on a backpack for commuting, get something waterproof. At least this way you can logically justify spending up to $150
on a school backpack.
Seriously though, do yourself a favor and get a top box. You'll love yourself (and us for recommending it) for it.