Desperation or Vantage for Tharja?

Life and dead + desperation or fury + desperation it would be the best for your tharja I think, I don't know personally I prefer to put vantage to melee unit or units with close/distant counter. For mages I prefer desesperation or sword/axe/lance/bow/tome braker.

Tharja will most of the time one round kill most units she face if buff correctly. Desperation will be best if you combine it with a fury o LnD+ reciprocal aid/ardent sacrifice.
If you don´t have a good buffer for blade tome I would recomend to keep vantage for a while.