I think this would be an easy decision for most people because the curricular differences are so significant and obvious. However, there are some other factors to consider:
1. Exam style. For pre-clinical classes, Penn quizzes students once a week--usually on Mondays or Tuesdays--whereas Columbia administers holistic block exams every couple weeks. Columbia's biomedical program makes for a more rigorous academic education, but it also offers students more flexibility in the weekly schedule. Thus, you are more able to take weekends off at Columbia. However, Penn's structured program paces you so you will not have to cram as often. For clinical years, I think (correct me if I'm wrong, here) evaluations are competency-based at Penn, while Columbia simply requires the satisfactory completion of x number of each procedure.
2. Dual Degrees. At Penn, MAs and MPHs are free and are completed at night within 4 years. At Columbia, you have to take at least an extra year to complete an additional degree, and I believe you must pay tuition.
3. Student culture. It appears to me that Columbia students are more cerebral, laid back, and have interesting non-dental experiences--very much Ivy-League types. Penn students are more professionally-oriented, competitive, and value success through hard work moreso than raw talent. Despite both being private Ivy League schools, I like think of Columbia as a laid-back Harvard and Penn as the distillation and combination of the best qualities of "less prestigious" schools; they are the best in their own different respects. This is totally my subjective opinion, though.
4. Personal safety. Probably the same at both schools at this point in time. However, if the current political status quo is maintained, Washington Heights, Harlem, etc. will continue to gentrify while Philly will remain a dump.