Single point connects to the rifle at a single point.
Two point connects to the rifle at two points
Three point connects to the rifle at two points and has a strap that runs along the rifle and connects one end of the sling to the other.
Determining what you are using your rifle for pretty much determines your slings.
For combat, service rifle, three gun style shooting a combat two point sling is the best. This would be the Blue Force Gear Vickers sling.
For sniper, precision and target style shooting you would want to run a two point sling like the TIS M24 or Turner style slings. I haven't used the style Longshot is refering to so I can't comment on it.
For some applications the single point sling is the way to go. One example would be for breaching shotguns where it's only used a certain times. It can also be used for three gun for those that don't like two point.
Single point slings work very well however they do have downfalls.
The three point sling is on the way out as other slings like the Vickers slings can do everything the three point was supposed to without the disadvantage of the strap getting in the way of the bolt catch, charging handle etc.
I heard alot of reference to Isrealis and other forces about what they are using, however if you compare the combat experience of most of their troops compared to the Americans, Canadians and others in Afghanistan and Iraq you find a huge difference.
I also see alot of guys slinging their rifles the wrong way. The best way to do it is over your strong shoulder and under your weak arm. You will see photos of Isrealis, reservists and others doing it the other way, mainly with three point ( but also with two point) slings. This is so they can rest their arms on the rifle while standing around.
Take a close look at what you want your rifle to do. This will help your choice in slings.