Quality of the material and stitching is the main difference. I would imagine the two main types of single point slings are the bungee type and non-bungee. Most people here would say that you cannot go wrong with the Blackhawk bungee style sling. I've seen another very similar sling at the shows for about half the price. I would say having a quick release buckle just above the rifle hook is most important, it makes it so much easier to put the rifle on and take it off. I think I bought my sling directly from the rock river website (non bungee style). I would also say that most people on here would say make sure that whatever you buy is not an airsoft rated product.
As far as the sling mounts, just pick the one you want, there are a couple of different designs, left handed, right handed or ambi. Think mine was about $10 (pro-mag).