This is actually a very tough question(for me) I have so much respect for both men. Royce is a legend, sure somebody else may have come and introduced BJJ to the world but, the fact is Royce DID! That being said, I would ask what are you in particular looking to get out of a seminar? Instruction from one of the most proficient men on the planet? or having the privilege to share the mat with a legend? Your original question was "who" I would have to go with Marcelo, but Royce deserves the utmost respect.