The Reptisun 10 tube is what we use for our custom enclosure, because no matter where our dragons go, they can always get uvb (unless they're in their cave). With the Mercury Vapor, they recommend it for dragons that desperately need uvb. Like dragons with MBD. It focuses on one spot though, so it's not as good at getting little bits of uvb everywhere. It's more for one direct spot with a little bit of filtering. So if you are looking at giving off a ton of uvb in one spot, then use the MVB. But, personally, I recommend the 10.0.
That's a personal preference. We use the exoterra sun glow for our silkie because she needs lower wattage and then we use flood lights from Wal-Mart for the other two. They produce a ton of heat, and you get two for ten bucks. You just increase or lower the wattage based on what you need. Our open tanks need more heat than our custom, so they have higher wattage bulbs.