I am late to this thread, but I highly insist on going for power rather than Ninja. Cave keys have become more important due to the introduction of Charms as they scale together coupled with other things.

Sailor Sakura is indeed a great Ninja, but in the end you must decide power or ninja. Also ask yourself, what teams will I be running with this ninja? Buying ninjas without a proper team in mind is simply throwing away your hard earned coupons or money.