I'm looking for suggestions for a backpack to hold a 14-15" laptop and a change of clothes. My commute is short (5 miles) and ride a road bike with no braze ons and have no need for a rack. And if I get a bag from Timbuk2 that is around $100-130 range I can get my job to buy it for me so that limits my choices.

I was looking at this messenger/backpack.

Ace Laptop Backpack Messenger Bag

I like the fact that it converts from messenger to backpack but then I see some cycling specific ones:

Madrone Cycling Laptop Backpack

My main question is what makes a backpack "cycling specific" and what advantage would I have with that one over the other if any?