5 star upgrade

I recommend you hold off on upgrading either of them, as the Falchion wielder's get a refinement upgrade tonight at 11 pm so you might want to check that out to see if it'll change your decision.

Olivia doesn't need to be 5*'d unless you want her Level 40 5* conversation so it's either Eirika or Marth.

I'd go for Eirika so she can unlock her prf Sieglinde. Olivia is generally better merged as 4*+10 since she doesn't need the Silver Sword+ unlocked to be useful

Eirika depending on what the new Falchions bring. Her sword with a dual rally and a buff in her C slot makes her a very good support unit.

EIRIKA always. I mean, you have to focus alway son Fire Emblem Sacred Stones. Its the best game and deserves the best.

Ok now yes, I would take Eirika 5*. I mean, she gets her Sieglinde and its buff effect. If you have Roderick then you can buff all stats in a single turn. Do you know how good is that? Specially if you have a tomeblade user. Your Eirika also has the best IVs if possible (for me).
I use my Olivia 4* and I never had problems with her. All she needs is her Dance command. And Marth... im not sure, I really prefer Eirka afer all.

i have a 5* olivia and shes great. can tank the hell out of green mages. stick an armorsmasher on her and shes good for armored units as well. only worry is blue mages, but she should be positioned out of range of them anyways. not to mention theres plenty of her available, so getting her up to 5*+10 eventually is not out of the question.