I'll add a few thoughts. Those who like to reload on the run want a horn up high under the arm where it isn't swinging around. A horn that is so low as to rest over the flap will swing around if on its own strap.
Some folks say a horn over the flap is in the way when rummaging through the bag, I've never had that problem.
If your bag is made of "hard" leather and the horn is swinging around you will hear the horn tapping against the bag. On the other hand if your bag is soft leather or canvas the horn over the flap conforms to the bag, doesn't swing around, and keeps the flap shut-which eliminates the need of a button on the flap; and, offers quick access to the bag.
So...if you walk and don't reload on the run, and have a soft bag, a horn hung on the bag's straps and positioned over the flap is great. If you reload on the run or have a separate strap on the horn, a higher position keeps the horn from swinging around and tapping on a hard leather bag.