should I promote +Atk -spd or +spd -hp sothe?

+spd -hp is probably his best IV set, he needs the speed to consistently double enemies

+spd definitely. He has good enough atk already, and that little spd boost from the boon helps him secure more doubles, even without buffs.

Most people will recommend +spd -HP, and it is a good IV, but I can tell you from experience with a +HP Sothe that +atk -spd (harder-hitting Kagero with more bulk, basically) is also a very reasonable IV, except you won't be playing for doubles unless your opponent is slow. +spd -HP it probably isn't better than -def or -res because he's very unlikely to survive two hits without healing in between.

I would probably go for +spd though. Better survivability when at full HP, and he's the only dagger that can have such high offensive stats. Kagero can compete with him in atk, but he's better when going for both atk and spd because higher BST improves his bulk and everything about him