I've actually thought about this quite a lot, been thinking about doing a fairly large one myself and here are some things to consider (assuming the dimensions of the opening are the same regardless of door type):
- one large vertical door give more room for moving things in and out than 2 horizontal doors, unless removing them is an option for large accessories - sometimes that extra inch makes a difference
- 2 horizontal doors allow access to one side or the other at a time and would work better to maintain temp and humidity in the set up and could allow for having the inhabitant kept on the closed side while working in the open
- what's being kept could dictate the best answer, If you're dealing with animals that stay put, it's a matter of convenience and aesthetics, but if you got darts or something else jumpy, being able to keep in what belongs in would be tricky with too much open space
- you didn't list it but I've thought alot about a door within a door, maybe one large hinged door and then a cut out in the glass large enough to reach through for feeding and what not with a second piece of glass used to overlap the hole, I've seen it done with magnets and acrylic - pretty sweet and 'best of both' sort of thing