Equating liberal arts with the humanities is a common misconception, but I agree with your basic premise that some majors will entail more work than others.
[“Liberal arts” is an umbrella term that includes math and the sciences (biology, chemistry, geology, etc.), the social sciences (anthropology, geography, psychology, etc.), and the humanities (philosophy, religion, literature, etc.). It usually includes the arts as well, though BFA programs are a different beast.](http://img02.deviantart.net/8020/i/2010/323/9/3/the_more_you_know_by_stathisnhx-d33639v.png)
Engineering is, of course, not under the liberal arts umbrella.
I meant to address this earlier. Princeton is uniformly awesome at pretty much everything it does, of course. Duke is similarly top 5 or 10 in several of the disciplines it offers (public policy, biology, religion, classics, environmental science, anthropology, English, etc.), so there’s little or no difference between the two. In some other disciplines – such as math, astronomy, and East Asian studies – Princeton has a noticeable edge; likewise, Duke has some strong majors that Princeton does not (e.g. statistics and dance). The answer to this largely depends on your son’s interests and career goals, but for most students there’s not too much difference.
The requirement of a senior thesis and the inability to double major (though many programs offer certificates) set Princeton apart. That may have more of an impact than any difference in academic heft, to which undergrads are frequently oblivious.