1 points are best suited for when you are hanging around a limited area such as a shooting range or operating in an indoor environment. Anytime you are walking you end up holding the gun with at least one hand.
2 points are better for traveling longer distances ie patrolling or hunting. You can let the gun hang hands free while you walk.
3 points are good if you like screwing around with your gear. I personally don't like them.
That said, I have found that the light weight and small size of the PS90 (assuming you don't bolt a bunch of crap on it) allows for the use of 1 point slings where other weapons such as the AR or AK normally require at least a 2 point. I do a lot of hunting and walking in areas with dense brush, creeks, hills and some bottom land. I use the 1 point SG sling from Vestguy and it has performed well for me.
I started out with the 1 point Urban ERT sling. He then later debeloped a 2 point method to use it after a few months. I got the necessary upgrades, and have used the 2 point system since. I LOVE it. It is the way to go. U can still hang the weapon downwards (with the barrel pointed down). It works great this way, and is more comfortable than a 1 pt setup, IMHO
two point slings in general are the way to go. they give good muzzle control when transitioning to a sidearm or kneeling and you can switch to a weak hand shooting grip relatively easy. they also help carry the weapon on extended hikes better.
the only pro with a single point is that you can transition from strong hand to weak hand relatively easy, other than that they suck. they do nothing to keep you muzzle out of the dirt if you go to a kneeling position.
three points slings suck the most. yea they carry the weapon okay but you can't transition to weak hand without choking yourself.