Bilbao are indeed the favorites to win the Copa del Rey trophy this season, because they were able to beat Barcelona and ATM and reach the final. I think Mallorca chances are somewhat lower, even though they managed to show different performances in La Liga and the Copa del Rey. Overall Bilbao managed to balance their performance, and currently they are trying to penetrate the top four of the La Liga standings.
Whatever the position in La Liga, Bilbao at least has one ticket to make them as a winner. Yeeee, Bilbao vs Mallorca, I am pretty sure that Bilbao will win the final match of Copa del Rey. If you look at the composition of the squad and its matches so far, Bilbao really has greater strength than Mallorca. but somehow, Mallorca's strengths in the Copa del Rey and La Liga are quite different. Even in La Liga, they almost occupy the relegation zone. But in the Copa del Rey, they entered the final. This is not luck, but this is indeed a very strong effort for them in the Copa.