vertical makes more sense becuz all sides of the bulb are exposed to the grow area/plants where as horizontal ur only reflecting one side of the bulb. in my opinion its wasting light distribution. i have a dome on my 400watt MH for clones and i love it. i use 1000watt MH for my mothers and 1000watt HPS on a light mover for flowering but they are both horizontal and i need to purchase some "dome" reflectors for them but like i said my 400watt is awsome>>> at the moment i am using my 400watt for my mother&clones becuz i dont have the room for all 3 ballasts running at once, but i have 12 adult female plants growing under it along with a bunch of clones so it just goes to show how important a refector is.
heres a pic of everything as is right now. (setting up the 1000watt MH when i take down the 1000watt HPS)