In my opinion power is more important than level, but it's really easy to fall behind on things like strengthening equips...

Concerning what warhubs said about seals - I'd save 'em for Kage treasure, since Kage offers a lot of ninja that you can't get through elite instances, like Torune and Fu. And as you probably already know, you get coupons when you get more ninja.

That said, don't just try and get characters you don't need. Once you get a nice rare from the Kage treasure, I'd stop and save until the next treasure comes out.