I am a member of the 600vert club. I dont have a lot to go on as far as horizontal growing. I grew under a 8bulb t5 for a couple of years. Then I lost half the bulbs (ballast burnt) and went to a solis tech 600 digital. (awesome ballast btw)
For me, going vert was cheaper. I didnt have to buy a hood. Ballast, bulb, and socket. The room is 8x7x7. I bloom 8 plants in 3.5 gallon hempy buckets using Advanced Nutrients 3 part ph perfect line. 2.5 zips average per plants dry. ( I say average as i run 3 strains one is a heavyweight that gets big). Now i could increase my plant numbers and proly get better then 2g/watt, but i grow for personal and dont need it. I dont train, top, supercrop, lst. i do trim the plant after stretch to get rid of popcorn and anything growing on the backside of the plant.
I personally dont see a point in horizontal grows. Hoods are heat traps and you lose lumens with glass and reflectors. Barebulb has better penetration and imho is easier to cool.