What is more important: luck or fighting spirit?

I think that Damage Block (DB) is more important because double attack will be more interesting than double damage – the bonus can be used more flexibly. But in general, I don't like these skills and I would never take them voluntarily. If it's really necessary, there are artifacts and places on the map where you can increase both, and instead of these skills, it's better to take, for example, attack or defense – there will be more benefit.
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The only exception is the Castle faction; given their focus on DB, this skill can sometimes be useful.