Horizontal i.e. parallel to the ground. I like to see over my broadhead, otherwise my eye is drawn to the vertical back of the head and I don’t want that distraction. Horizontal also prevents pulling the back of the blade into your hand, potentially, should your arrows be cut that short or you over-draw. Flight is the same either way, some would argue otherwise. Spin test always, a non concentric head (wobbly head) will impede flight and penetration. Try both, make your own deductions.
Kris