Question on awakening mystic 4* champion.

AcuTarA

In the past event I managed to get a 4* mystic awakening gem. I have my 4* Ghost Rider, Dormammu and Magik already duped. I was saving the mystic awakening gem for a future champion like Scarlet Witch, Doctor Voodoo or Mephisto. I just got a 4* Scarlet Witch and wanted to ask you guys which champion do you recommend to use the mystic awakening gem on, do I use it on my Scarlet Witch or wait until I manage to summon Doctor Voodoo? I have 7 generic 4* Signature stones and 7 4* mystic signature stones, so not that many posible Sig levels for SW. I don't know these champions very well, if you could explain the basics it would be greatly appreciated.