PENN usually has between 20-25 students from california in each class. Almost all of them want to come back to Cali and practice and PENN has a great reputation in getting you to where you want to go. If you want to specialize you should consider PENN since almost half the class goes on for post graduate training. This year we matched 14 people into Orthodontics which is unheard of at other schools.
USC is a good school however it is more expensive than PENN, also it is PBL which you should look into prior to making your decision. If you think PBL is for you then USC maybe your choice, however you can't go wrong with PENN.
If you want more information you can go to our forums and talk to current dental students at PENN at: