Which Nowi for +10?

Is this for Arena? If so, I'd stick with +Def - Spd, speed tiers are pretty weird in general towards the higher merges since bulk matters a lot more (you get more bulk the more you merge since you're adding more def/res and hp compared to just atk the higher your merge, so enemy phase units shine in high tiers).

+Def -spd. It'll be more useful during the enemy phase.