Shizune is okay if you want a high level wind pet in a short amount of time (because she is burstable and doesn't take exp away from fires). However, if you want a strong pet.. Shizune is not the right pet for you in my opinion. Yes she's fast and useful for sealing, but her attack hits two mobs (therefore, it is weaker) and she is a hybrid as people have said.

I would go with Hayate for a strong pet. I have a level 108 Shizune and a level 50 Hayate. Hayate hits stronger than my Shizune... soo xD