- Build a pressure vessel to withstand the differential to the atmosphere
- Let the helium do the pushing from the inside but make the thing 14% bigger
I don't see how the vacuum ever wins.
For one, if you have a pressure differential, you need to care very much about even the tiniest leaks. A surprising fact about airships is that they don't actually have to be very gas-tight to work just fine. As people found to their surprise in both world wars, you can shoot an airship full of small holes and it only very slowly degrades in performance.
3 ways.
- Vacuums aren't combustible.
- Vacuums can be created without sourcing a gas.
- Vacuums allow you adjust buoyancy without requiring you to carry extra gases.
The major downside of a vacuum is finding something ridged enough, light enough, and cheap enough to maintain a vacuum.