Personally, I would go for the shrimp. You can give them algae wafers, or even drop in a few extra pellets and force them to sink to the bottom. I would just try to get some of the bigger shrimp, so they won't easily become snacks. My ghost shrimp are huge, and about the same size as my female betta. She hasn't touched them. Chases them sometimes, but she hasn't actually attacked them. My male betta, though, has nipped at a few of the ghosts. They're fine and don't seem to mind too much.
~ P o s e i d o n ~