Just my thoughts, as I've got various sorts. For a short CQB gun then a single point on a sling loop behind the receiver is probably the most comfortable. For a sniper, standard two point sling at butt and fore-end is best. Intermediate (i.e. M4, L85, Aug, etc) then either a single point or a three point would be best, especially (as with the issue SA80 sling) if you can unclip it and use it much as a single point to increase your speed of aim but re-attach if you need to push it on to your back when using a pistol, etc.