Another Arkel Commuter user here. I've been using it for over a year and have had no problems with either laptop damage or weatherproofing (optional cover works very well). I am able to carry a 15" laptop, charger, large lock, change of clothes, towel and shower kit and still have left over space in the bag. However, there is not a lot of internal organization options (spot for laptop, then the rest of the space is open).

My concern with the Arkel briefcase would be your heel clearance (since it appears to be quite wide). Be sure to test the bag on your rear rack and bike before making a purchase. A bigger rack may solve clearance issues but would be an added expense.