Not that it matters much, but the answer is VODKA, with the caveat that neither are as superior to water as you might imagine.
StarSan concentrate works by lowering the pH of the water its added to. However, when you add StarSan to any water that isn't completely distilled, and is exposed to oxygen, then the StarSan starts breaking down, and the pH of the solution will slowly rise to "normal" levels where bacterial growth isn't inhibited. If your StarSan solution gets cloudy, then that means that its no longer sufficiently acidic to work as a sanitizer. If you put normal water + StarSan into an airlock, within a day or two it will be cloudy, and therefor only as effective as plain water.
Vodka has the advantage of being 40% ethanol, which will inhibit bacterial growth as well, but it will also slowly loose its ethanol to evaporation over time. I'm not sure how long this will take, but eventually you'll be left with nearly-pure H20.
Food-grade glycerin is the best choice if you want a liquid that won't evaporate out of the airlock after a few months of neglect.