Tough one...
Priscilla:
PROS: Anima magic, mounted, needed to recruit Raven, is the only troubadour in this game,
CONS: Gets the mounted weaknesses, is annoying to recruit, later recruit chapter, is accused of incest
Serra:
PROS: has a way earlier recruit chapter, has funny supports, needed to recruit Erk (I usually get Erk before Priscilla),
CONS: has lower movement, has slow supports, has the weakest magic, is sometimes annoying,
I know my PROS and CONS suck but this is a small way how I see it. I voted Priscilla anyways.