That is a dilemma we'd all like to be in. I'd say first come, first served. If they are small indie artist I'd let them all record it. They are likely not going to cross paths unless it becomes a smash hit country wide. Some songs get covered hundreds of times. Once it is out there, anyone can cut it as long as they pay the royalty. Big name artist may want an exclusive on it, but unknown artist don't have that luxury. Good luck with it.