There is quite a debate going in the photographic community, which one is more comfy: back packs or shoulder bags. This has been going on every since I can remember and talking strictly from a functional perspective I can see pluses and minuses for both. (if you base your decision on style, which is perfectly ok, then this will not matter to you).
Shoulder bags have the benefit of easy access to your gear, while backpacks are easy to carry around. But what if you could have both. Paxis has a nice nifty bag that does some which is as close as it gets.