Which weapon upgrade should i get on Nowi

Neither cause it seems your nowi has no IV nor skills.

Depends on your build and IVs, however I would actually say +Def or +Spd. Unless you’re running Vantage or perhaps TA the extra MT isn’t as useful as the other refinements.
Now between Def and Spd, I would go Spd with Fury and Def with Steady Breath.

Hope this helps.