I'm not sure if there's a whole lot of difference between them. As long as we're talking about real messenger bags and backpacks. FWIW, I use one of Chrome larger bags (Metropolis?), and love it - but 99% of real messengers I see here in Berlin wear messenger backpacks.

If you're in a very hot climate, messenger bag might be better than the same capacity backpack, but for me the difference was marginal (when I lived and commuted in Tel Aviv).

Others may know more though.