Hero merge and dragon problem

I would say that it's best to feed the first Nowi to Ninian because -at iv is the hardest to recover for Nowi because she will pretty much never double anything without a breaker skill anyway (and she doesn't need a speed stat with quick riposte and built in distant counter). I never encourage ftp players to promote a unit just to give another one a skill (it's a massive waste of hero feathers that are better spent elsewhere), and even if you're a paid player, you have bigger things to worry about than whether or not your dancer can counter from a distance. I suppose if you still think that Lightning Breath+ on Ninian is a must and that -spd iv Nowi is somehow worse than -at iv Nowi, then feed the -spd Nowi and keep the worse one.