Single point is so you can drop your carbine RFN and transition to a pistol, or handle some other gear that needs both hands with urgency. If it wacks you in the nads or kneecaps you when you drop it, well.. that's just not important.
Three point slings are complicated (at first), but allow you to do several useful things.
1. Carry it "Israeli Style", slung at your waist, kind of like a purse for ready action.
2. Carry it half waist/half back, in almost the traditional sling position.
3. Split the loops and carry it muzzle down, "backpack style".
4. You can use the loops for a three point shoulder hold like the 1903 sling, or somewhat like the scout-loop/Ching sling idea and get your forearm/elbow in there so the rifle is braced at four points when shooting. Fore support hand. Sling on the elbow. Rear hand on the grip, stock on the shoulder.
Single point is what you want to be wearing for "trouble". Three point is what you want to be wearing for a patrol or long watch IMO.
Some of the best slings are at Blue Force Gear.
Just playing with a .22AR, I'd get whatever was cheap at Brownell's/Midway. To a degree, nylon webbing is nylon webbing. Fastex buckles are Fastex buckles. For a class or three-gun, I'd go with better, but barring bad stitching, I'd be kind of surprised if even a Chinastan sling actually failed.
Here's a nice tutorial on making your own.
http://theprepared.com/index.php?option=com_content&task=view&id=106&Itemid=55
