Why do you want a sling? There are two choices: (1) you want to carry your rifle more easily, or (2) you want to shoot better.
I bought my first sling before I knew the difference, so I ended up with a Viking VTAC. It's an excellent carry sling that provides some shooting stability in the standing position.
After taking an Appleseed clinic, though, I bought a half-dozen USGI web slings, because I decided I liked shooting my rifle more than carrying it. A good web sling (or leather marksman's sling if you want to go that route) is probably the best single performance accessory you can buy for your rifle.