To follow up my own comment: I suppose these aren't mutually exclusive. For instance, you could have an airship filled with hydrogen or helium at 1/2 atmospher

To follow up my own comment: I suppose these aren't mutually exclusive. For instance, you could have an airship filled with hydrogen or helium at 1/2 atmospheric pressure and kept hot by solar heating. For helium especially it might be an effective way to keep costs down (since helium is so expensive) and maybe keep it from leaking out quite so much. Similarly, using hydrogen at partial vacuum would mean having less of it on board, which is sort of good from a flammability point of view. Though on the other hand, it means that if the skin is ever broken you'd have outside air mixing very quickly with the hydrogen inside due to the pressure difference. That's probably a bad thing.