Decision: F!Takumi or Nina

One Vote For Nina.

Skadi may be a bit gimmicky but Takumi is a GHB unit so he'll be a bonus unit in AR and Arena from time to time. He's also a decent debuffer.

With those IVs Nina will have 2 less atk than Takumi which is a very bad thing as archers hid def that normally is the highest stat in the game making atk the most important stat for them, Nina still has 3 more spd but in the current meta, unless you're Ayra Karla or Lewyn, spd isn't the most important stat mainly because dragons (which are the most common and strongest unit type along with armors) will double throught skills rendering spd a much less usefull stat.
The defining factor for the choice, in my opinion is their phisical bulk, in arena and in PvE having enougth bulk to take a hit and enter desperation safely is an essential factor for their utility, both are able to enter in desperation from a mage but Nina will hardly get into desperation if the enemy hits her def while Takumi can.
If Takumi runs his prf he will have better offensive stats compared to Nina, since his weapon gives him +3 spd they equalize in that stat and he has better atk which is very important, and Takumi also has the better defensive stat being able to take a hit in both def or res

Why don't you suggest to use Clarisse instead of Nina (if you plan to use Clarisse's bow)? Nina has nice res but unexistend def stat if i'm correct, unlike Clarisse, her def and res stats are decent so she can become pure supportive unit that can heal ,debuff and do chip damage in one initiation. (You can do similar thing with Nina if you need res tank) Fortress def can be replaced by SS3 but having 33 def on both phases is viable enough to sacriffice 3 points of her atk.