I think its simple. If your family would have to take loans to pay for Harvard, then go to USC. If your family can “swing it” to send you to Harvard, but will have to stretch, then go to USC. If your family has significant wealth, such that they can easily write 300K worth of checks over the next 4 years for Harvard and then another 300K for medical school, and can do the same for all of your siblings, then go to Harvard if you like it better (sounds like you do).
Your ability to get to med school will be based on how you perform personally, obviously both schools have all that’s needed to get you there.