PAC bags are amazing and from my friends with RELoad bags, they are too. Seriously consider getting a backpack. The new(ish) RELoad midpack looks really nice but doesn't have base compression straps. I opted for the Freight Baggage backpack as a result. I got the medium but most of the time, I think the S would be sufficient.

A properly designed messenger bag is meant to be ridden while at least slightly hunched over. So wearing it on a scooter or motorcycle or while walking won't be as comfortable.