Which Nowi

Fury is optimal for the +spd. You can initiate and do more damage and potentially even double in some cases(refinery and seal).

However, if you eventually do get Steady Breath, the -spd is optimal for tanking the maximum amount of damage.

So, I would hang onto both and wait for SB fodder. Use an offensive Fury build for now, or maybe even invest in both.

I use a +spd/-hp nowi and honestly she's a monster that way. Fury Vantage QR seal destroys most units. She has no problem killing other nowi's. I especially love to see the SB nowi because she slays them easily!

What are you looking for in your arena team? Do you want to get to Tier 20 and rank up in AA? Or you don't really care?

You are very lucky to get these two Nowi. Don't merge away either one. Both have their advantages. It depends on your team. And your team is bound to change if you want to increase your score. (If you don't want to increase your score, there is no reason to merge anyway.)

As a temporary option, I would run the +Def one. Sure +Spd and Fury stack nicely together. But you have no merges, while your opponents probably do. So the stacking is rather incomplete. You don't have DC red units. But Sigurd will survive any green mage. So your Nowi shouldn't ever need to tank hits from green mages -- we are mainly concerned about Cecilia here -- and thus you don't need the extra speed to avoid double. You can pretty much do anything with A-slot on the +Def Nowi. Just be sure to give her QR3.