I have the medium bag and the backpack, two different colors.
I can attest to the fact that they both are very durable and can hold quite of bit of baggage, but you have to keep in mind that they themselves are rather heavy without anything even in them.
The backpack carries the most, can pretty much withstand anything you can fit into it along with the fact that it is carried in such a way that it makes lugging a heavy easy to carry. I tend to use the backpack in the fall/winter and for travel, the medium for warmer months. Both are excellent everyday bags that never show a mark.