Thiem. He's an aggressive baseliner who uses a lot of top spin and has a lot of back swing which allows him the time to move the ball around and find the open court or get the short ball. That is his principal game in a nutshell and one plenty of clay courters have built their games around. Think Costa, Muster, Ferrero, Kuerten, Moya.
Murray is not a Clay courter. His success on Clay is impressive, but not a result of him being a good clay courter. More because he is an excellent tennis player and their games do well on any surface. Thiem is going in that direction, too.
As for who is the more decorated clay courter, it is Murray. But we can't answer that question until both their careers are done.