Supers on gnw treasures are totally viable on their own.

Supers on kage requires skillbreaks to become viable and seeing the speed we get skillbreaks, maybe in 10 years we will see them.

don't worry, the whale only breaktroughs will appear very sooner than the others.

Anyways, for the OP: both the treasures are good, so is mostly up on your other ninjas. Itachi susano'o requires 2400-4800 seal scrolls used since you get 1-2 fragments every 60 rolls you do.

it generally depends on what you want. The problem with GNW Treasure is that the rare ninjas (every 60 scrolls) only work well with each other or other edo tensei units. Kage rare ninjas are more versatile (honestly didnt even try Kushimaru, Edo Sasori, Jinin, Zetsu, mangetsu, and i used Edo Zabuza for a little while. The only ninja that i use regularly from GNW Treasure was Jinpachi, Fuguki, and Pakura), but as shryke said, the super rares are WAAAYY better in GNW Treasure.

So ask yourself, do you want GNW super rares or not.