If I could only have one I'd go with horizontal, but it depends on what you play. When I first built my cab, all I cared about was playing Robotron and Moon Patrol. Horizontal was a must. Also, if you have a large enough monitor, vertical games are playable on a horizontal monitor.
Like others said, you're probably going to want a second machine eventually. I had my horizontal machine for a little over a year, and I'm working on a vertical now.