If you want to be relatively competitive on your server (regardless of if you ever beat p2w people) with other f2p people you choose power, period. You can focus primarily on power and still strategically plan which ninja you need to get to maximize your power's effectiveness.

Bottom line is, you can have all the ninja in the world but I will still stomp you into the ground if you don't have the power to compete with me. I've played this game since a week or two after it first came out and I have spent less than $500 in that time (with a spending strike during that time because of oasis' shady tactics) and i have put a majority of my resources into power while strategically picking up ninja along the way.

By being smart with my resources I have been able to crack the top 20 on S2 and I've been there for a while and I haven't spent thousands and thousands of dollars as some others have.