All matches between Barcelona vs Real Madrid are El clasico matches. Be it in the Copa Del Rey or the League, even all the games if they meet. The difference is just that the match in the League feels difficult to restore Madrid's chances of winning even though it wins because of the already large difference in points. While the best opportunity I think is in the Copa Del Rey. Madrid still has a chance to qualify for the final.
Of course, Real Madrid will try to win at least one tournament, it is not fitting for the royal club to finish the season without significant victories. They will try to beat Barcelona in the cup and the chances will be equal, as in the last match the game was very equal, until the last I could not give anyone an advantage. But Xavi will want to win all the tournaments in Spain to leave Ancelotti with nothing.