Well Medusa's damage rises quite a bit by dupe, but Corvus death immunity is a extremely handy abillity. Both are equally deserving of a gem depending on what content you need done. Medusa is average unduped, Corvus is a tier above her unduped but both become better awakened.
Well... Corvus will definitely help with Cavalier and beyond, but Medusa is key to more annoying opponents in 6.2 and beyond. Personally I'd do Corvus so you can push through Cav, and after that rank up Medusa for 6.2
I have both an unduped Corvus and a duped Medusa, and I wouldn't swap them. Medusa is mediocre unduped - awakening her makes a huge difference. Corvus awakened is really handy, but not necessary to blow through content with him.
Comments
Both are equally deserving of a gem depending on what content you need done.
Medusa is average unduped, Corvus is a tier above her unduped but both become better awakened.
Personally I'd do Corvus so you can push through Cav, and after that rank up Medusa for 6.2