Penn may be the Ivy closest in distance to Princeton, but how similar are the two schools? Raleigh Martin '08 offers a comparison.
I attended Princeton as a civil and environmental engineering major in the Class of 2008, and now I'm in my second year at Penn working on my Ph.D. in earth and environmental science. It's hard to make a direct comparison between the two schools because undergraduate and graduate education are so different. As an undergraduate, at least at a "liberal arts" school like Princeton or Penn, you take classes in lots of different departments, get involved in lots of different extracurriculars and generally explore as much as possible. At least that's what I did. As a graduate student, the emphasis is on delving really deeply into one topic (for me, sedimentology) and focusing on research.
Needless to say, there are major differences between Princeton and Penn. The first glaring difference is the environment. Princeton is in a suburban utopia, filled with manicured lawns and surrounded by multi-million dollar suburbs. Penn is in West Philly, right in the heart of the city. As a graduate student, I really appreciate all that the city has to offer, but based on my experiences here at Penn, undergrads here don't really leave campus. And I think it's generally true at all residential universities that undergrads don't get off campus much --- the vibrancy of campus life and extracurriculars is enough to keep you occupied most of the time. Therefore, I would argue that Princeton is a better environment to be an undergraduate --- a quiet, secluded ivory tower, and the perfect place to begin one's exploration of the world of ideas.
Another big difference between Princeton and Penn is the size of the student population. Penn has twice as many undergraduates, and probably five times as many graduate and professional students. Princeton is an "academics only" school --- no business, law, medicine etc. (Engineering could be considered an exception, but engineering at Princeton is so theoretical that I don’t consider it to be professional.) Penn has every professional school you can think of, and consequently the emphasis is tilted significantly in this direction. Princeton's small size and concentration of undergrads allow it to focus more on undergraduate education, and the courses for undergraduates at Princeton are certainly harder and more rigorous (and probably of better quality). That being said, if you are focused and driven to get an early start on a pre-professional education, then Penn may be a better match (though the lack of a business school at Princeton does not appear to be much of a barrier to the financial professions).
Socially, Penn and Princeton appear to be very similar in terms of the types of students they attract: generally upper-middle-class and hard-working, but also very socially attuned and wedded to the "work hard, play hard" mentality. Penn may not have eating clubs, but they do have big fraternities and sororities that appear to play a very similar function. I guess the one big difference is that there are more social options at Penn, whereas the eating clubs at Princeton often seem like the only game in town. Oh, and food and housing is definitely much better at Princeton.
If you're a former Tiger who is now pursuing graduate studies elsewhere or a Princeton grad student who was an undergrad outside the Orange Bubble, and if you would like to contribute a comparison to the Oranges and Apples series, please send an e-mail to email@example.com.