When you can't fodder Sothe.

Depends on their intended use. With +Atk you can run Heavy Blade, if you have an idea for some kind of special build, I guess. +Spd could be a Sweep de/buffer. Or, you know, whatever standard boring builds kids are using him for these days.

I think for more versatility or more variety of builds, +Spd is the better bet. It allows him to escape doubles and make some himself.

+spd by far. I myself had the same issue during the release of sothe and I went +spd. His atk stat is high enough that you don't really need a boost to your atk stat, your speed though could use some tlc and with a boon there you assure alot of kills he would otherwise fail to get because of his speed.

Speed. His attack is already high and with LnD + speed seal he can double just about everything.