Check out the Tenba line of bags. Built like tanks and the camera dividers (movable) can be taken out as a unit insert, so the bag can be used for other things, then slipped back in as a unit - dividers still in place.
I use the large messenger bag but the medium might work for you. Lots of pockets for misc stuff, too.
I can vouch for the quality of the canvas bags.
If you can go to a brick and morter store, look at all the bags. Get a feel for the actual size, capacity, weight, etc, and compare to the kind of shooting you will be doing. The decision, like so much in photography is subjective.
Good luck and have fun. (With that D7100, I'm sure you will!)