All slings suck. Some more than others.
Few folks use a "single point sling" more than once. At least not without a jockstrap and steel "cup" to protect vital family areas.
It's called "the pendulum of pain" for a reason... They do look kool posing for your carbine class photo wearing ACU pants a black polo shirt, and, of course, sunglasses.
Two point sling preference varies but the latest darling of the sling crowd seems to be rear of stock to a point just forward of the delta ring. Both on the side of the rifle. Second preference is front of handguards (where the M4 swivel is mounted) to one of those stock plate thingies that fit easily on the M4 but not on the M&P15-22.
-- Chuck