I do horizontal for veg cuz its easy to fit enough pots under the hood, and cus I have the hood might as well use it. But for flower I stack 600watt HPs vertically and surround the lights by two full circle of pots. Smaller 3 gallon pots that I'm running white widow in in the front, and 7 gal pots with critical mass behind the 3 gals.
Works quite nice, the difference in pot size keeps the back plants high enough that the 3 gal pots don't block light.....