If you are a f2p, power should always come first most of the time. I know that f2p can't beat p2w, but still you need to go for power in order to stay competitive among other f2p. It will allow you to get rewarded in a long run (gnw,swb,matsuri,convoy/plunder,space time). You will also get seal and summon scrolls as well as coupons from these events which will result in earning more ninja from treasures or buying them using coupons. Only go for ninja if the events offer no or really weak power items or stuff that you have no use for. Exception should be that you c*e that ninja for several months or strong end game ones.

Eventually, by being patient, you get both power and ninja at your disposal while also remain competitive.

Power = Rewards from events and competitions and later on treasure ninja and premium ninja

Ninja = You just get to play with newest characters before other f2p, but later on competitive f2p will surpass you in power and NINJA.