From what i've observed so far, the advantage to single points is that you can switch the weapon from shoulder to shoulder without taking off the sling, and also its easier and faster to detach the sling from the weapon when needed.
Disadvantage is that it the weapon has a tendency to flap around some unless its in your hands, since you pretty much only have one option to how it will be hanging, namely with the barrel pointing at the ground and into your legs.
The 3-pointers seems more comfortable, since they offer some more support to take the weight off the weapon, and also you can move that weight around when needed. And also you can pretty much just let go of the weapon, and it will be hanging with the nozzle facing in angle you left it at, very nice when kneeling down and similar, since you wont be pointing the nozzle into the ground by default.
Disadvantage is that its hard to detach in a hurry - since they have a tendency to fold out to a full yard(meter)'s length, when unbuckled - I'm sure i dont need to explain how anoying running with a yard of line attached to your weapon(if you get so far, since it probably will be filtered onto your webgear when you released it), and also, unless you made some half half 1 point/3 point system, you cant really move it to the other shoulder in a hurry.
Speed and simplicity versus a slow and comfy i guess
Im sure one of the Pro's can give a better explanation though.