If you just like collecting stuff and don't care about pvp that much, go for ninjas.

If you want to compete, and do spend money, go for OP ninjas.

If you want to compete, but is f2p, go for power, ignore ninjas. You have to live with what ninjas rng bless you with, that is the price of f2p.

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OP ninjas will let you beat people of higher power for the time being; but when the meta shifts and the ninjas you have are no longer OP, you'd fall behind. Spenders can simply buy more ninjas, or spend on power to not be behind in power. For a f2p, meta shift means if you focused on ninjas, you are now with low power and no strong ninjas, resulting you becoming irrelevant competitively, and will remain so indefinitely (if you don't convert to a spender.)