They are going to try to use the establishment clause as an argument and I think the Supreme Court (if this case makes it that far... I hope it does!) has the right mix of judges to rule in favor of the Town of Greece.
But think how much that would cost the town to contest the issue that far. Notice that the language used by the plaintiffs employs financial threats. That kind of bullying is often used by the militant secularists even when they know they might lose the case.