I run the Specter Gear MOUT sling because for competition and classes, it is extremely convenient to be able to snap in and out of the weapon without removing the sling (or snap a new weapon in). I've been very pleased with it; but I don't know if it would meet your needs.
The Blue Force Gear SOC Sling is a pretty neat set-up. It is basically one sling that can be rigged in any conceivable fashion (single-point, 2-point, 3-point). So for a little more than the cost of a good quality sling, you can get a sling that will let you try out all three methods and decide which suits you best. If you go for the SOC-C Contractor pack it is also configurable for about any weapon made instead of being AR-specific; but it costs about $30 more.