Well, you need to figure out if you want to go single point or three point, or a different type altogether. Single point and three point (or all variations of the "tactial" sling theme) are mainly used to retain the rifle when in use or during handgun transitions. If you are looking for a sling to steady the rifle when shooting, then a tac sling isn't going to do it.
A single point sling allows the gun to hang straight down and weak side transitions are easier. Depending on how you get the main loop to hang on your body, you can swing the long gun left or right and allow it to hang under an arm. However, it does allow it a greater degree of flop when you are moving, ie running, and can get in the way of your transition to a pistol if the long gun hangs on the same side as your holster. You will also need to get some sort of sling loop plate to install the sling on your gun.
Three point slings hold the long gun muzzle downward, but at an angle (ie muzzle point down towards your left side and butt near your right shoulder, or vice versa) making it ideal for holding it at ready or port arms for extended periods of time and because it holds the gun somewhat more static, it is easier to move in (like running) and easier to transition to a pistol. However, since most three pointers have quite a bit more webbing due to the design, many users feel that there is inteference when hitting the bolt release, weak side transitions are slower (or impossible without removing the sling from your body depending on design) because of the more static hold on the gun. Also, some people feel that all the webbing makes them all tangled up with the gun.
Look through all the pictures on Specter's website on all the different types of slings and pick the one that looks like what you imagine you will do with the rifle. When you have decided on one, but still can't figure out attachment points, use, pricing, etc, call them and ask. Their customer service is superb.