Sorcerer is the best class for Tharja to end on. The Dark Knight does not allow her to equip dark magic tomes and the extra mobility does not make up for the lower damage output. That is not to say you should avoid the Dark Knight class, though. Lifetaker is a useful skill to obtain, but if you do not want to get that specifically there is not much reason to use the Dark Knight class. Whether you are planning to reclass to Sorcerer or promote into it first, it allows Tharja to demonstrate her true magical potential.