Which sounds better in your room, to your ears? That’s probably the way to decide the better orientation. 

Ok thanks @nik9669a. I was just wondering if anyone was aware of a major differences in the orientation before I order the horizontal or vertical wall mounts. 

Just a bit of a guess, but I would think horizontal might have a wider sound stage?

I have a pair as music speakers with one connected to my turntable and the other connected to my CD player.

Unlike a lot of folk, I have them horizontal as I found I preferred the wider soundstage compared to the more normal vertical orientation.  

For some listeners there is no audible difference between the orientations, for other listeners there is a major difference. Only you know what sounds “best” (to you). If anyone insists that you are “wrong” just ignore them. It’s similar to food or wine tasting. There can be tasting events where some self appointed “expert” attempts to discredit anyone who disagrees.

If, on the other hand, you don’t like the sound, someone with more experience can suggest some solutions.

Do try them in both orientations before ordering stands. Make yourself temporary supports to hold them as close as to where you intend to add the mounts.