If it isn't obvious enough, I am a sling manufacturer.
If you want a great sling at a reasonable price, check out my site. You can also IM or email me with any questions you have.
http://www.rockymountaintactical.net/
We make and sell slings to LE/military, hunters, shooters, etc...
To answer your questions about the different types of slings out there...
Single points are used primariliy for weapon retention. They are most effective when you are in a scenario when you know you will need to have optimal maneuverability with the weapon and know you will need to use it, like for PSD, dynamic entry, needing to shoot from both shoulders (like around corners), in a vehicle, etc...
With a single point sling, it should be mounted near the rear of the receiver, so you may need to buy a new sling mount to install there.
They are great for situations when they are needed most. They are not a good all purpose sling, as they do not allow the user to go hands free because if you let go of the weapon it will hang there and bounce around against you when you walk/run/climb. They simply don't provide the ability to secure the weapon to you tightly so that you can use both of your hands, whether you are running, climbing, securing prisoners, carrying wounded, etc... or anything else when you need use of both of your hands for things other than the weapon.
Single points provide weapon retention with the greatest freedom of movement with the weapon. It's likely that for a carbine class, you'll either want a single point sling, or a quick adjust 2-point.
2-points are good for going hands free, but they CAN be too restrictive... that is why a quick adjust is so useful. It almost eliminates the need for any other type of sling because it is secure when you need it to be and in an instant it can be loose and unrestrictive.
3 point slings are over complicated and have too much excess webbing. I would avoid them.
Having a single point and a two point will cover all of your bases. That is why we have developed two slings that are capable of multiple configurations that utilize those configurations.