Nowi and her IV +Atk or +Spd discussion

+spd is simply more defensive since she will be doubled less often than with +atk and so better for a "bait"

I still prefer running +atk since it give her a bit more OHKO potential.

EDIT :
I see this as
+atk better player phase
+spd better enemy phase

I consider nowi to be an enemy phase unit with the upside that she can preform well during the player phase too.

I also believe that she is more suited for tanking than dishing. I would also say that she is very easily killed. Speed prevents her from being doubled by a lot of things that could potentially kill her. Spd is probably also better used with fury, whereas if you are going TA, raw attack might be better.

You feel +spd is not the best? As long as arena's concerned, you are 100% right! Japanese player are playing -spd nowi for a while now... and it's the best, probably with +atk. 33 speed(considering you put fury on her) with a speed boon is trash, she'll get double by everything relevant. I would merge my +spd into my +atk.

If you don't play much high ranked arena then that's a different story, +spd has more value then, but even then I don't feel it surpass -spd +atk, as more content now is 40+ and units either has ridiculously high spd, or very low. With support you could reach decent speed though, maybe enough to avoid double.

I'm in the same boat as you; I have an 5* +Atk Nowi that I use, but I also have an unleveled 4* +Spd that I've been keeping around. Both are -HP. I haven't found a reason to choose +Spd over +Atk.

I run my +Atk Nowi with QR3 as an enemy phase unit. Whenever I do the calculations with +Atk or +Spd for a QR3 Nowi, it doesn't seem to make much of a difference. The way I saw it, her speed isn't her strong suit anyways, even with a boon, so I opted to take the Atk points instead.

However, I'd say to hold onto the +Spd one in the event that Hone Dragons comes out. +Spd Nowi would reach 36 speed with it, and I find that a lot more persuasive in "avoiding doubles" than 30 speed.