I would think an LLM in tax from any approved program would be beneficial. I don't think there is a hoard of people dying to do tax law, but taxes will always be there and are only getting more complicated so if you know and for some crazy reason enjoy tax law people will pay you to help them deal with it. I would think LLM would be the most beneficial law related degree you could have. That is just my opinion as a law student though and I am sure someone has more info than me.