How should I spend my next 20k feathers? 5* Nowi, or Aether for GRobin?

What it says on the tin. I'm building a dragon team and so far my only 5* dragons are Myrrh, Ninian, ATiki and my brand new GRobin. I have a 4* Nowi with meh IVs, and I'm not sure how she'd fit into this team. She'd probably be the better blue choice offensively, of course, but I like having a dancer. Fortify Dragons is nice, too.

So I almost have 20k feathers again-- should I upgrade my Nowi? (Who'd she replace on the team? Ninian? I like Myrrh and Robin both so much, though...) Or should I upgrade a 4* Chrom so I can give Aether to GRobin (because it looks really good on him)?

Obviously I would like to do both eventually, but which one is the better idea to do *first*?

edit: My GRobin is +Def, so maybe Ignis would be a better choice for him anyway??