Which is more important the keyword density or keyword prominence? I've see website that has greater density which is 2% above while their prominence is less than 50% and their website rank position in firstrank in google.
Keyword density is a myth but like a lot of myths you can still find some people who believe in it. Just write good and appropriate text for your users and forget about keyword density. Keyword prominence means different things to different people, what exactly do you mean when you use the expression? - Michael
I wouldn't spend much time worrying about either. It's not really a case of how often you use a keyword, but where. Add to this, on page factors are no longer as important as off page factors nowadays.
It's not a bad thing if you give some time about that matter. But yes, mattuk is right you should focus on how you will use keywords. But to give you examples, I've seen articles ignoring the keyword density rules. About keyword prominence, those are keywords having a very stiff competition.