I think both would be good too. However Serenity is just joining the Titans. But It may also depend on who is active and can post a lot. (I have no life and I'm on all the time XDD)

Edit- 

Serenity could help with a lot of the Titans problems but she lacks authority really and the ability to be mean when it's required. She would always find a way to do the kind way. While Armelia is bold and not afraid to do what gets done. But she has trust issues so I think she would try and do the hard things on her own. Both have good qualities of a leader and their own problems.