My opinion of vertical growing is that it will always outyield horizontal growing when both are dialed in. The reason is simple: when you hang a bulb vertically and place plants all around it, most of the light being produced is shining directly on a plant, maximising its effects. When you hang a bulb horizontally, you will notice that only about a quart of that bulb is shining directly on the plants below - the rest (75%) is being reflected off a hood.
There is an equation where for every doubling of distance, there is a four-fold reduction in lumens (exponential), which means if light has to travel twice as far to reach a plant, it only carries a quarter of its original photons. If you draw a line from your bulb up to the reflector and then down again - where it reaches the plant - you will see that the light has to travel at least twice as far - which quarters it's output.
There are some advanced hoods on the market now that claim to be about 70-75% efficient - that is, they reflect 70-75% of the light produced. A vertical bulb is at least 80-90% efficient to begin with (a small amount of light escapes through the top and bottom of the bulb that doesn't reach the plants). Furthermore, once that reflector gets dirty, the surface loses its ability to reflect - this is compounded in sealed-glass reflectors, as the glass gets dirty and filters the light (that's why I don't use cool tubes - they get dirty easily).
The other advantage of vertical growing is you can keep the bulb very cool by blowing a floor fan on it - which allows you to place the plants very close to the bulb. Hoods and reflectors capture heat. You can duct that heat away, but the metal hood will conduct at least some of the heat and stay warm. Plus with a bare bulb, you also save money on reflectors, ducting and fans.
Finally, vertical bulbs allow you to take advanatge of head height if you have it. So you can use a smaller footprint for the same-sized grow, or you can simply grow your favourite sativas - like I do - without having to worry about them getting too tall.
Would you believe I actually veg under a horizontal light? That's only because I don't have much head height in my veg/cloning chamber, and I am not so much concerned with rapid growth, as I have 8-10 weeks of flowering time to clone and veg before my new plants start to flower.
EDIT: Here's what I'm talking about with light hoods and light distance:
