In my opinion, you should go with a backpack.
Past experience tells me that shoulder bags (I think that's what you mean by bag), while they may look better, do move around a lot and when you are trying to shoot from a lower angle, they swing around.
Capacity is not an issue as there are quite a few bags that will hold your gear. Although something you might want to consider is comfort. Shoulder bags put all the weight on to one shoulder and if you shoot for a long period of time, your shoulder can begin to feel uncomfortable. Backpacks, however, spread the weight across your whole body (if you use all the straps).
I suggest you look around at the lowepro website:
Backpacks - http://products.lowepro.com/catalog/Camera,14.htm
Shoulder Bags - http://products.lowepro.com/catalog/Camera,20.htm
And go to a store and try them out....Most stores will let you put you gear in there and try them on.
My suggestion on the backpack is either:
Lowepro Flipside 400 AW or,
Lowepro Mini Trekker AW
As always, if you end up buying over the internet support the site by buying from one of the affiliate stores.
Hope this helps,