Lol, none of them I am running a bare bottom setup and another one with just some handfuls of gravel, but if I had to choose one I would pick gravel, sand is often sucked by the filters and it can damage impellers and even the motor well in hobs, but it also depends on what kind of filter are you using. If you are running canisters or even a sump then you would have less problems with sand damaging the filter mechanism just place the intake tube as far as you can from the sand surface, if youre running hobs then I strongly disadvice sand.
My two cents.
Cheers.