In this case I will go for Real Madrid as for betting. I think they will be one step ahead of their opponent in this case with the help of Benzema and Vinicius especially.
Xavi is very experienced as a player. But he is inexperienced as a coach. He has just started his coaching career. And being the coach of a big and star-studded team like Barcelona is a bit of extra pressure. Even then I'd say Xavi has been pretty successful. The Barcelona team is now stable. Yet occasionally we see poor performances from Barcelona. Xavi will try his best to win the Copa del Rey match. Because they will be playing against their arch-rivals Real Madrid. Both Madrid and Barcelona are in good form right now. So it is difficult to predict which team will win. Ancelotti is more experienced than Xavi. And so I would put Madrid slightly ahead.