+Atk/-Hp or +Spd/-Def Nino?

-Def +Spd is generally ideal.

You will get a lot of attack through Gronblade, so I would go for the +spd for more chances to double and safely ORKO with Desperation. Nino won't get much mileage out of def so you're not losing much there.

for sure +spd due to the fact that she can get enough atk through her weapon and as well as buffs and other skills. not getting doubled and doubling the enemy is vital to nino, +spd for the win

I've personally been rocking a +atk/-Hp with Summoner Support, and she hasn't let me down(unless the other units on my team got killed by BK in TT.)