Muzzle down. I find it MUCH faster to the shoulder and very natural movement.
For the sling it self, There is three I like. the Ole standby Military sling is tough to beat all around. the simple Marlin strap sling that used to come with there rifles is also great and as I say, simple. Lastly a bit fancy, Uncle Mikes Suede "flared" sling. it sticks well to the shoulder and being slightly padded is quite comfortable on a long days hike.
CW