I've got both sides set up vertically. I need to be able to see both rear tires and also down the road so vertically works best for me. On the right side, I've got mine set up like Xfaxman. The problem with that is when the windshield dirty, I can't see through the windshield to the mirror well enough, and the wiper doesn't get that low on the windshield. I've got a wired 5" camera/screen that I bought at Costco on the center of the dash to see straight back. For some reason, my center mirror is mounted very high on the windshield and even with the mirror down as low as it will go it doesn't give me enough clear field of view back down the road.
If only design engineers would ask long time customers what upgrades they'd like to see, they'd have a much better product. And that goes for any piece of equipment, from Toolcats to dishwashers.
DEWFPO