Which Ninian is better?

A speed bane is bad since Ninian isn’t exactly tanky, but so is a defense bane because her defense is weak to begin with. I’d say speed is more important than defense in most cases, but you seem pretty screwed no matter which way you go since a 30 speed lets a large number of doubles happen, and dropping to 20 defense makes it an autoloss for her against anything green, dragon killing, or Brave. Just pick one and refine whatever breath she uses to fix her bane, or slap a seal on her. I’d say go with -def and just slap a seal on it.

Ninian has good speed for a dancer (Neutral Spd Ninian can reach 42 Spd if you refine her weapon, give her Fury, and a Spd+3 seal.) and low defense. Since she mainly plays a supportive role, avoiding doubles is the main way she will stay alive rather than having an extra 3 Def. I'd say use the -Def one.