What is more important: luck or fighting spirit?

I had a question that came up when I saw it being discussed here – does the morale (luck) stat actually have an effect if it reaches a value higher than +3 or lower than -3?
It seems to me that morale is less stable than luck; that is, it can be very frequent at +3, but sometimes it "stutters" and doesn't appear as often. Or am I just imagining things, and it's just a matter of chance?