Astolfo or Fionn?

Well your decision I know a few guys who'd raise Stheno without hesitation, so....In my opinion both are surprisingly viable after their strengthening quests which you need to wait for year, but if you can Fionn is much more of a support and one time tank for arts crit teams I don't know who's good with him, but some say it's Nursery Rhyme and Tamamo. His NP is AOE, but even after his interlude his NP gain is only average in my opinion so he still isn't that good, but more viable only.
After his interlude Astolfo is one of the rare riders who could charge half of their NP gauge and that says a lot together with Ozy as support then he could probably a clear mobs and become a stronger crit servant, but he isn't really that stable due to his long cooldown.
Well I for gameplay purposes would just pick astolfo after his intelude due to his crit ability with Ozy and his ability to charge his NP.

Do not burn them. Just prio whichever class you have least of.

Astolfo because he's cuter.

Astolfo gains some pretty significant buffs in the future for his NP and even his 3rd skill which adds in an NP gain as well. He can be useful for wave clearing and farming and even as a survivalist in some tough fights thanks to his NP's evade.

Plus he is cute as mentioned above.