Unless Penn is offering you significant merit aid or you absolutely hate Chicago, Chicago is the correct answer here. Discounting the new USNWR rankings as the garbage they are, Chicago and Penn are not peer schools. Chicago will give you a significant advantage in any competitive career path in any market. I don't think slight preferences in environment should be enough to make up for that. Take a look at the Ciolli and Leiter firm placement studies and the lawfirmaddict data...it isn't close.
But I imagine it is getting closer, because of the quality students that base their decisions on US News' rankings and who conclude that Penn is truly even with Chicago. In this way, the law school rankings make the law school rankings.
Penn was 12th in the rankings as recently as Y2K. Penn's rep scores and LSAT numbers have always been lower, even significantly lower, than Chicago's. So Penn games the rankings, ties Chicago, and waits for its rep ratings and class GPA/LSAT numbers to catch up to its overall ranking. Makes sense.
Of course, we are splitting hairs. This stuff only really matters for clerkships and academia placement and for people who finish near the bottom of their classes grade wise. Most people don't need to worry about that stuff. Choosing based on vibes and perceived fit with one's personality is most important. Good jobs come easily from either institution.