Celica is cheaper to build and easier to fit in a team, because she doesn't rely on buffs that much. My absolute favorite way is running her with both Distant defenses (A skill and seal) to bait and usually just kill whatever attacks her. Renewal is a safe bet as a B slot to make her even more self-sustaining and threaten res (or speed) to weaken her foes before fight.
Tharja is a typical blade mage with all its strengths and weaknesses. +spd IV is highly preffered over +atk tho, so you could try waiting. She's still usable and strong with +atk. She will not fit into every team, because she's reliant on her team to buff her and/or save her from possible threats.
I'm adding my builds so that you can sort of compare the potential stats. I still need to grind sp for threaten res for Celica tho and I'm thinking of swaping her special for draconic aura.
You can always swap it whenever you're not using BK, which is the huge advantage of those seals. Yeah I forgot to add that Draconic Aura would fit better into the LnD set that she can run as well (yeah I probably like her too much, so I'm running two separate builds on her). In the LnD case, I'd say that draconic aura is better and in normal distant def build keeping Luna will be fine.
I would personally use Celica because i like her as a character more even though her iv's arent as good. My tharja is +spd -def while celica is +def -atk. The better is definitely tharja because of both her boon and weapon (blade tome op). So tharja may be the better unit to go with but if you really like celica you could give her the blade tome and that would be ok as well. Personally i like to use characters that i like.
For example i really like Florina. Is she great? not really but i built her up anyway even though i have hinoka and tana which are far superior to her.
Heres my tharja havent built her yet but just to give an idea of her stats.
I mean, I have a +spd/-res Celica and a +spd/-hp Tharja and I wouldn't say that Celica is OP. To put it like this, if Celica is under 100% hp Tharja is simply better. So for PVE (TT, GHB, BHB...) I would say that Tharja is better as she uses Desperation. Sure, you could put Desperation on Celica, but what's the point of using Ragnarok then? And if you're not using Ragnarok, why use Celica? Her offensive stats 32/33 are just not high enough to justifiy using a bladetome.
For Arena I would value Celica more than Tharja, as she doesn't need buffs to function well. Ragnarok compensates for that. And, while I don't think she's OP I think Celica objectively is the best red mage. On that note, I don't understand your build. How do you get Celica back to full HP?
As far as promoting Tharja goes, I wouldn't do it untill you pull a +spd one, 34 is just not enough for a bladetome mage in my opinion.
Overall, I'd have to say that I'm not terribly impressed by red mages. It's simple. The most common green mage is Nino, and unbuffed Celica can't one-shot a buffed Nino if she doesn't have Life and Death 3, and if she has LnD3 there is a possibility of Nino OHKO-ing her if she's buffed and merged enough. Tharja is even worse, even though they were on defensive tiles I've killed a highly merged Tharja multiple times easily with my Nino. In other words, Celica and Tharja have limited enemy phase use due to their low res and for player phase they are outclassed by other infantry mages such as Nino, Delthea and Linde (Celica is equally good as them, but only with full hp).
I'm a huge fan of using mages and have used them a ton, as can be seen from my Hero Merit list, so if you think I can answer another question you might have feel free to ask.
Thanks for answering my question too :)
I personally use both Renewal and Reciprocal Aid on Celica (https://imgur.com/a/4LxLk)
I had a +atk, -hp celica, and a +atk, -res. Tharja. My celica with LD3 hits like a tank. But I found that my Tharja does equally well(I have her paired up with ephraim and Axazura for buffs) and sometimes even better when she is fully buffed. Either way, both are extremely reliable units. I feel like it is simply a matter of team building. You have to build a team around Tharja because she is a blade tome user(and less flexible than nino or linde), but celica is much more versatile. It's simply a matter of preference as I find both are very usable. I simply use Tharja more because she is more versatile with buffs (I have two buffers on my team), than celica is without ragnarok.