Experience is definitely important. You have to know what this game is and how it works to do well. General games mechanics, and previous knowledge gained from your past mistakes is paramount. Everyone started at the bottom, and they all had to learn the hard way. No amount of guide reading can make you a good player.
That said, Adaptability is also important. Being able to analyze a situation and tactically decide the best course of action can be the most useful tool to a player.
Where experience gives you skills such as timing, splitting noble trains, defending, attacking, village builds, troop builds, etc. Adaptability gives you diplomacy, creativity, leadership, charisma, and so much more.
You may be the best attacker out there, but if I can convince you to leave me alone for a week and I get all the tribes around you to attack you, who is better then?
I don't necessarily think one is Much more important than the other, but they definitely compliment and supplement each other.
In the end, being a good diplomat is useless if everyone just wants to attack. And being a great attacker and experienced player won't get you far if you can't convince everyone to back off for a while.
So in conclusion, I think that the best player is a player that can strike a fine balance between the two.