I read this question in a WoW blog recently and thought it was pretty interesting.
Arguably, the two are remarkably similar. Both are ambitious, ruthless leaders whose main concern is the advancement of their own race. Neither really care about the other races in the Horde (though perhaps Sylvanas cares a bit for Elves, which technically are still her own people). Both of them use whatever despicable means necessary to achieve victory. Both of them poison the land around them, and both use chemical weapons or weapons of mass destruction. Both torture. They both have made huge gains for their people in terms of territory and power.
So, why the difference? The only difference I can really see is style. Garrosh is loud, stompy, and angry. Not very subtle. Sylvanas is cruel and cold, but more quiet, subtle and calm. What do you guys think? Or maybe you hate Sylvanas too? Or like Garrosh?