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.
In La Liga it is difficult for Real Madrid to catch up to Barcelona, the Copa del Rey is one of their big opportunities to become champions. The Champions League competition is very tight so it's not easy to determine whether they will be able to win again or not this season. Maybe now they will not focus on chasing Barcleona in La Liga, considering that the chances are very small, we will see them perform completely in the Champions League.
Yesterday's result in the Copa del Rey surprised me. I found a comeback from Real Madrid quite possible, but Benzemá's hat trick and a score of 0-4 in Barça's stadium Camp Nou was very hard to predict. They should not have much trouble defeating Osasuna in the final.
La Liga is a different story: it is almost impossible to close the gap between them and Barcelona, so they will (should) focus in the Champions League, where, although difficult, they still have chances to win.
The more interesting question is after that marvelous win yesterday whether Ancelotti can keep his job when he wins Copa del Rey and perhaps the Champions League. Or whether he would step back himself after another big success and say "that's it my friends". I could understand that move as he had an outstanding career already. He said that Real Madrid will be the last club for now. Maybe he wants to try again next season, He seems to be physically healthy and he is only 63. Some will say that is quite old, but he isn't the wild guy anyway and comes across as quite relaxed most of the time. He could definitely handle to be a coach for another few years.