From my experience horizontal is the way to go. Basically my set up is hope trials hub, revell frame and an avid ultimate. I put it all together tightened it all up, got the tension perfect with chain tugs and that's it. I haven't touched it. I run maxxis tyres so I don't need to worry about punctures, but they wouldn't be too much hassle really. My wheel NEVER moves, my tension is always perfect. I can't see why you would want vertical drop-outs. Take a look. The above is an old pic from when I had a king with a quick release (that didn't work too well!). But just look how tidy it is. I think it's the best way forward personally.