I am actually from Boston, and I have worked at Tufts. I also knew many students who went to Tufts undergrad, which perhaps sways my opinion of the school in a negative way. Tufts is located in Chinatown, but that itself isnt really that bad in Boston. Tufts is also extremely expensive. California sounds much more pleasant (ignorant as to USC's location), so I would probably choose USC over tufts based on my very subjective reasoning.
As another poster mentioned, I only know USC for its football, and my impression (lets see how wrong I am) is that its a large school with lots of West Coast kids that is about average. I don't think being a graduate of either medical school would give you an advantage over a better performing student from another. 3.6 vs. 3.4; what does that even mean? (I know its a residency director ranking, but srsly - could that just be a result of a lower STEP I average at USC?). Just some food for thought.