My betta is kept in a 15 gallon planted community with sand substrate.
Sand is fantastic. As far as the betta goes it seems to make no difference one way or another. The betta could care less.
Sand is great though. Its density means that no food or debris will sink into it where it will decay, like gravel. Mulm is lighter then sand, so by hovering a syphon over the sand bed it can be picked up with minimal amounts of sand. In smaller bowls and tanks airline tubing can be used as a vacuum, simply syphon water into a bucket and hover the tube around the surface of the sand.
That I think sand looks 10x more natural then gravel, is, of course, my own personal opinion.