What it comes down to for me alot of times is whether I want to grab out of the bag and shoot fast, or can take off a backpack and take my time. I don't particularly like large shoulder bags with everything in it, so I try and use something small for a body with attached lens and use a lens case on a belt. I have also used a larger waist pouch as a bag for the lenses. I usually pickup used bags as they are usually so cheap and I can see how much room there is and figure out a configuration. You can also get Domke inserts for a standard backpack that work very nice.