I would need a better estimate of the monetary differences between the two. Else, I assume your support system (family and friends) are located in CA. Combined with the better weather (in my opinion), unless your goal is matching at a big name East coast residency, USC is likely a better choice.
Additionally, if I remember correctly, Dartmouth's base loan increases every year, while USC opens up more scholarship opportunities as you progress. So while the initial difference may be in favor of Dartmouth, USC might match or surpass their aid over the course of your four years.