looks like the 17" is a pannier and the smaller one, 13", is just a bag.

btw, the inside of bag has an image that some folks found offensive. i think it added whimsy to an otherwise black bag. i liked the bag since it also had an easy off the bike carry option - a wide, comfortable strap that makes the bag look messenger bag-like.

that said, as folks have said, people love their arkel bugs. i too considered this but since i wasn't sure of what i might actually carry i was afraid it might be too small.

oh.. another thing, i do notice, even on my short ride, the difference between a grocery pannier and the knog. when i have my laptop and gear in the knog, the bike feels more balanced than it does with the grocery pannier. it's likely two things.. 1. the grocery pannier isn't really aero. it is just a square box 2. the knog has the laptop compartment close to the rack and the way the bag is shaped it sorted forces the cargo to be as close and tightly secured to the rack as you pack it.

pros n cons as always.