Use a nice fine-grade gravel. Bettas appreciate a smooth surface to rest on when they are sleeping. I don't suggest sand. I'd be too worried about them eating food off the bottom and injesting the sand. I know when I was breeding them, it was emphasized not to use sand as a substrate. I imagine it's just better off not used at all with bettas, but that's just me.
Also, just a tip.... make sure there's plenty of plants or other hidey-holes for the bettas to go to in order to 'ignore' each other for a while. Constant exposure to other males can cause them to become distressed, losing color and clamping their fins down submissively (which if done for too long permanently clamps their fins... believe me, I know!) If they have hiding places, they will flare and fight for a bit, then go away to 'chill out' for a bit. It keeps them happier that way and it's more like their natural environment where they exhibit that sort of behavior. That's my two-cents' worth. ^.^