Ceteris paribus, I would select the student with both the higher GPA and higher PCAT score.
The PCAT score is a way to equivocate/standardize everyone's educational background.
If the Harvard graduate with the lower GPA would have scored higher on the PCAT, I would have selected him/her.
It is important to note that the magnitude of differences between students play a role. If the margins are great, it is a no-brainer who should be selected. However, if the differences between two students are minute (a half of a tenth of a point separates two GPAs, PCAT scores are within a few (lets say up to 5) percentiles apart), then I would think that the interview would determine who is selected.
That's why it is extremely important to take your interview seriously. If a student with the highest GPA/highest PCAT score shows the admissions committee that he/she is not taking it seriously, it may hurt his/her chances. However, it is also important to note that each school assigns different weights (percentages) to the importance of the interview. Some schools value the interview more than others. Thus, I would try to do my best at interview, regardless of what position I think I'm in.