Which Weapon upgraded for +Spd -Res 5*Nowi

I have the same IV Nowi and also use Fury. I've got all the upgrades for her and, for me, +spd works the best. She's already bulky enough and +spd with your fury 2 gives her 35 speed, allowing her to avoid more double attacks which can be crucial.

Is this your only Nowi? What is your general situation? Why no Fury 3? Nowi is more than worthwhile for Fury 3. Also to be sure, you should never inherit a Hinata away at 3*.

First of all, if your Nowi is a temporary build, using Spd refine can compensate the lack of QR3. You can get other refine choices later when you have more resources to seriously build your Nowi. Refine Stones are not nearly as bottlenecking as rare skills and feathers. Also before the details, why Glimmer over Moonbow? You are not proc'ing Glimmer fast enough to assist in killing fast mages. For everyone else, either it doesn't matter or Moonbow nets you more damage.

Now let's talk about the more serious cases -- where your opponents are +10; cav emblems are spammed; and your Nowi build is now permanent, with access to at least QR3.

In general, I don't think highly of Spd stacking on Nowi. But if you have +Spd IV, +Spd refined may not be a bad idea. Let's talk about Spd stacking in details.

Speed stacking splits into two cases: 1) high speed player phase units who thrives off doubling enemies; 2) medium speed balanced units who can use extra speed to avoid double. Otherwise, when you are not speed stacking, you are speed dumping.

Nowi falls into the second category and potentially entirely out of the speed stacking categories. You are always going to be doubled by red units, bow units and daggers. And you don't care. You are also unlikely to double blue units even if you stack speed -- especially if your Nowi is unmerged. Again no big deal. What can be threatening is mages who can double you. What can be extra threatening is a blade tome Cecilia who can double with a Hone Cav buff. (To clarify, stacking spd further with Fury is NOT a viable option in the long run. So Nowi is never in the first category. Fury being not viable is because if you run Nowi at +10, there is little reason to not use Steady Breath given the existing investment. Without Steady Breath, Nowi struggles in many ways. If you are at +0, you are already 4 points behind in each stats compared to your opponents. All Fury does is to patch those points back.)

Now a Nowi won't just exist in vacuum. She is going to have companions to look after. So blue mages can typically be dealt with by a DC green unit (e.g. Fae) or a DC mix tank blue unit (e.g. Fjorm). Similarly, green mages can be dealt with by DC red units. A common problem, however, is that you need your red unit on the team to be an offensive player phase unit. (I am assuming 3 units of choice plus a bonus unit in the case of arena.) And you may not want to commit a Hector just to allow for a DC red unit or you may not have a heavily merged Ryoma (who is otherwise terrible for scoring without the merges). So that's a common situation where you may find yourself needing Nowi to at least survive a round from Cecilia. A common tactical scenario is open map against a cav emblem team, in which case your Nowi may still have to tank a hit from Cecilia even with a DC red due to the movement advantage of the cav opponents. Thus, unless you run DC red and/or your player phase nuke is of green/blue color, dumping speed fully is risky.

Now I understand that you use Tharja. With Tharja, you are essentially treating Nowi as another armor unit. You need to think through what your third teammate is going to be. Depending on what s/he can do, your optimal Nowi can be +Atk, +Def, and +Spd. Also think about the kind of teams you are going to field in the future. Tharja and Nowi just don't synergize that well. Tharja is also not that good for scoring given her rarity.

In any case, due to the limited movement of Tharja, just like on other teams, I would suggest that an ideal Nowi should have enough speed to prevent double from Cecilia. You need to check the following 3 situations yourself and decide on suitable speed: 1) +Spd with Hone Cav buff; 2) neutral Spd with Hone cav buff; 3) neutral Spd with no Hone Cav buff. For fast mages, you need to focus on having enough Atk to OHKO them. Back to Cecilia, obviously you can carry Panic Ploy in Nowi's C slot -- if you have a Valter to spare. (Nowi requires DD seal.) But if you have to rely on Panic Ploy to survive any kind of Cecilia, you are bound to lose your deathless round to Cecilia frequently due to Panic Ploy not aligning. Also, remember to assume +10 on your enemies. For yourself, check the cases for both +10 and your current merge number. Decide what is ultimately optimal and temporarily optimal. Make a compromise between the two. As a bonus of being able to tank Cecilia and maybe other mages, you will have enough speed to double slow armor units and some healers. That allows you to use Nowi in player phase to finish off crucial targets, which is a major plus.

After securing enough Spd, if you have excess points to allocate to other stats, I personally would do Def because I would use DD seal on Nowi. I have yet to find a common case where Nowi needs extra Atk. (Such a common case will necessarily be for OHKO a mage and thus preventing a double. This can matter for you actually. Since your Tharja doesn't deal with blue mages. And so Atk can work for you.) When you can OHKO fast LnD mages or Fury Nino, you don't need extra Res. DD will cover other situations (e.g. Reinhardt and Lyn) and give Nowi the sustain. Thus Def is my stacking stat for Nowi (after sufficient Spd is secured). All in all, my Nowi is +Def/-HP with +Def refine. (Think of neutral Spd as already allocating points to Spd in Nowi's case.) I don't use Fury. I use Steady Breath instead. So unlike Fury, that's +4 Def and no more Spd. My Nowi can tank Reinhardt well before +10. Other mages are dealt with by Fjorm. (+10 Fjorm can deal with Reinhardt too. But heck, who would throw out so much money out of the blue. I wish I am that rich. Anyway, without the merges and without Glacies, since Ice Mirror is required for scoring absence of merges, Lancebreaker kills my Fjorm) Only Firesweep Lyn poses a threat. (which she does to every other team except for full armor.) Kagero & windsweep dagger combo would annihilate my team. I can only count on its rarity to avoid it. You can work through what Tharja's weaknesses are like how I complement the coverage between Nowi and Fjorm. You are going to have major weakness with movement. You also seem to seriously need a player phase nuke. But the plus side is your Tharja does counter Poison Dagger.

Lastly, -Res IV is detrimental on Nowi. Again, your specific situation matters. You need to decide how Nowi is going to be used. In the long run, -Res is not viable.