Originally Posted by MoAlpha

Having been a bike messenger and used a variety of shoulder bags on bikes in the decades when men wore such things on the street, my advice is get the backpack. Shoulder bags are fine on upright bikes, but as soon as you try to use one in anything remotely approaching an aero posture, it slides off your back and finds a highly stable location hanging off your side with your knee banging it. The pros use musettes for obvious reasons, but they have to sit up to keep them behind.

Plus, I mean, really????