ive had Nagato for a while and in my humble opinion he only works in teams centered around him and usually u have to give up other things for that. if i could exchange him right now for refines / charms id do it.

I actually upgraded Danzo to 4* today with lucky board and that gave me about ~600 power, but danzo is a support ninja and raising his stats wont have much of an impact. Danzo can do his barrier, chaos, immobile and izanagi just fine at 3*

I would suggest go for power.