What is more important: luck or fighting spirit?

I wouldn't have such a choice because, as a rule, I already have both Leadership and Luck by that point. And if we talk about the essence of the matter, I choose by trial and error because I consider both skills to be equally valuable and useful – it's always nice to get an extra turn or deal double damage. And I also believe that it's more reliable to rely on skills than on artifacts – what if you urgently need a slot? Maybe I'm wrong in thinking that both skills are equally valuable, but don't worry, when playing as Conflux or Necropolis, I won't take Leadership.