Should I Recruit Deen or Sonya?

Truthfully, you can go wrong with picking either Deen or Sonya. Sonya comes with great base stats for her level, and she has decent growths that allow her to be one of the best Mages in the entire game. Deen, much like Sonya, has great base stats and growths for his level; however, his level is much higher than Sonya, which means he's much stronger from the onset and he'll remain that way. If you simply regard both of them to their Classes, Sonya is a little more useful than Deen (unless you make Deen into a Bow Knight much later in the game).