Well, it looks like you are constrained to a field (genetics) at pitt, while you have more freedom at USC. Also, 30k/yr for two years for free (fellowship) is huge because it's good money, and you don't have to waste time with TA or RA and focus on your studies. The stipend of 2.5 seems much better. Plus LA has better whether and I think you will have more contact with collaborators at a school that has a better medical school (that's how it is at UCLA).