Get a Magpul sling that can be either single or two point, and decide which set up best suits your own needs...
That being said, If you have to choose between a exclusively Two point or single point only sling for some reason, I say go with a two point.
Its only downside is that its a little slower when transitioning the rifle from shoulder to shoulder, like when firing from cover.
Other then that a Two point does everything else better and wont hit you in the nuts if you have to go to your pistol, or drop the rifle to the sling and it can be slung and carried in more ways (across your back...etc). The slower transition time of a two point can be minimized if you experiment with where you attach it to the rifle. (l like mine attached to the end plate and the other point attached to my hand guard, behind my fore grip.)
A good way to think of it is a Single Point is more of a rifle retention system then a sling. A single point sling keeps the rifle attached to you more then it carries the rifle.
A two point will both keep the rifle attached like a single point, but will also carry the rifle if you need to use both hands.