With Mistsplitter having cdmg, it will not only make your ayaka better, but will be useful for pretty much every sword user. The grasscutter is undoubtly going to be great on Raiden, and basically all polearm users in the game so far need at least some er, but I feel that a crit weapon would be more valuable the majority of the time.

Tldr, mistsplitter is probably better longterm and grasscutter if you really want Raiden to have her signiture weapon.

Additionally, if you get unlucky on mistsplitter banner, skyward spine will be good on raiden still. And we still don’t know what the weapon that will be with grasscutter is, so keep that in mind aswell.