It's two different competions and should be treated as such. What is more difficult win a marathon or a sprint?
For eg, in league you have only few quality teams who will challenge you, while in cup you have almost all who can chalange you in the knockout stages. Obviously the luck plays more in cup, but it can go both ways, you can get an easy path or you can get the most hard one, plus you will almost always will get really tough opponent in semis or final.
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It depends, in spain atm you really can't slip it seems(if you're not barca). For barca and Madrid it is definetly easier to win the league then the cup. But for a team like Atletico I think there is more chance to win the cup then the league.