Both are, of course, great schools. Here are things that seem relevant for me:
1) I’ve read somewhere that of the cross-admits, approximately 80% choose Yale and 20% Harvard. According to LSAC data, of the 241 offered admission, 189 decided to enroll at Yale. Of the 817 offered admission at Harvard, 555 decided to enroll. While other people's decisions shouldn't be dispositive of your decision, assuming that many people who get into YH are rational, it seems like this should tell you something.
2) Is there a chance that you want to pursue academia? Yale has over twice as much success in placing their graduates for prestigious law teaching jobs. http://www.leiterrankings.com/jobs/2006
... hing.shtml. I first thought this might be a little bit skewed, since it might be influenced by students' career goals, rather than placement ability. But if you look at the per capita Supreme Court clerkship placement rate, Yale consistently places around twice as much there as well - assuming that no student with a sane mind will turn down a SC clerkship, I think it's pretty safe to say Yale is superior over Harvard in securing law professorships.
3) Faculty: check profiles of faculty (articles published, citation rate, etc). I would say both are approximately equal - there are more of them are Harvard, and Yale offers an intimate setting. Faculty ratio at Harvard: 10.3:1; at Yale, it's 7.4:1.
4) Boston v. New Haven. I think many find Boston more attractive. I’m guessing this is accounts for part of the 20% that decides to go to Harvard.
5) It’s definitely much harder to get into Yale than Harvard. Interestingly, there are two transfer outs at Harvard and none at Yale.
Also, for a good source of appraisal of the two schools, ask current law students / faculty who do not have a conflict of interest at Yale or Harvard.