2 stars or 3 stars pb an't that much different.

but 3 stars Kushina will give her immunity.

I already have PB long ago, for me I'd go for Kushina, but isnt free charm better in this case?

EDIT: Or you can go for Kage Tsunade you'd get total of 5 frags for 10 boxes, 7 days.

That means you need another 15 events to complete her. But you would miss out 17 Charm Boxes (and Summon runes).