Do you plan to spend any time in Manhattan? If so, I'll offer a dissenting opinion.
I would take trains between Philadelphia and Princeton, and between Princeton and Manhattan (Megabus between Princeton and Manhattan is another option), and try to schedule the visit to West Point on a day when you can take advantage of the Avis/Budget $9.99 one-way special between Manhattan and EWR/LGA/JFK (Sundays and Mondays only), or between the airports and Manhattan (Thursdays and Fridays only). The special rate includes 12 hours, 150 free miles, and the first tank of gas. No special rate code is needed; just make sure that you search for a rental period that does not exceed 12 hours, and that you book at least 24 hours in advance.