I can tell you backpacks are a lot more comfortable to carry around while out and about.
I have a commercial aw lowepro bag which I personally prefer. Its great being really big (can fit a medium format and two 35mm bodies with several lenses for instance), but being a shoulder bag also pretty much a dead weight. The commercial aw isn't available anymore, but the same kind of thing is. I've forgotten the name of the new version. Apparently the new ones can even take a laptop in the top compartment. But unless you seriously need the space, I'd recommend a smaller bag if your going to wander around a lot with it.