Point well taken by midatlmom.
The NJTransit bus is indeed an option. However there isn’t much research going on at the UMPC at Plainsboro hospital. There are opportunities for you to do basic volunteer stuff at UMPC Plainsboro, but no decent research. You will surely have opportunities for research on the Princeton campus, but these will be limited to basic-science type projects. Although these can sometimes be impressive, you would be better off doing research in a lab with a more clinically oriented focus. The closest medical school is located in New Brunswick NJ, which would require a train or bus for you to reach.
Duke on the other hand would require you to simply walk across the street to find an entire hospital filled with clinical MDs with lab space.
Both schools are great choices, so you can’t go wrong.