Honestly, between the two I would probably go with Elhain. I think she's more all-around useful.

However, to be totally honest, I don't love either option. Elhain was my first 60, but she very quickly became relegated to the bench.

Do you have any other interesting epics to max first?

All the other epics I have are; Shaman, Zargala, Fahrakin the Fat, Jizoh, Aina, Chancellor Yasmin, Relickeeper, and Atur.

Fellhound is difficult to gear, so I wouldn't recommend him as your first 6*. I'd go with Elhain. Zargala is nice but would have trouble farming 12-3 as most of the waves are Spirit champs. 

Elhain is fine as a campaign farmer, i have her as faction wars only now but she can do brutal farming just fine.

Usually  you  six  star  your  starter  champion.  I  didnt  because  that  was  back  when  the  battle  pass  was  in  the  game.  I  didnt  buy  it,  but  on  the  free  side  you  coudl  get  six  star  legendary  gear  and  my  five  star  Kael  was  perfectly  capable  of  farming  12-3.  I  six  starred  Tayrel  and  Apothecary  first.  Elhain  is  your  priority.

I 6 starred Shieldguard and lifesteal on him.  I use him to farm 7-6 Nightmare and have not had a problem yet.  He is uncommon so getting his skills up is pretty easy to do

I would go with Relic Keeper over Elhain because I don't like Elhain.

NOT relickeeper, I don't like elhain at all but she is 100x better than rk. Put relickeeper in vault and don't take past 40

Post  your  roster,  will  make  it  easier  for  community  to  help.

You  may  not  have  many  champs  yet,  being  only  2  weeks  old,  but  it  generally  helps  people  give  advice.