I started off carrying vertical on the front of my belt opposite side from my gun. I then worked for a department that dictated where everything went on my duty belt and carried horizontal on the same side of the belt as my gun. I actually liked the horizontal carry a little better. I was also faster at reloading from an upright position because I didn't have to turn my hand over like when drawing a mag from a vertical pouch opposite side from the gun.
I am starting to carry a open top, 3 mag pouch and have been trying to retrain myself to vertical carry. Its a bit odd still but I'm getting there. I still prefer the horizontal carry but its doesn't work to carry a triple pouch.