Can't decide between a backpack or a sling? The Tamrac Corona 26 allows you to choose as you shoot. One of the backpack straps can be removed and tucked away, with the remaining strap then fastened to the opposite side for crossbody wear. The bag has access to the camera, ready-to-shoot with the lens attached, from both sides and the front, so you can easily access your gear whether you are right or left handed or wearing it as a full backpack. The bag is roomy enough for a couple extra lenses and the top compartment offers a big space for extras. There's also a laptop pocket.
The downside? If you fill up the bag to the brim, it will be rather heavy to wear as a sling. Most slings can only fit minimal gear, but filling up the Corona will make you want to use both straps--and possibly even purchase the extra waist belt. All said and done though, if you want both a backpack and a sling without buying two bags, this is it.