I think physiology would help you out more in the long run. I learned alot more in my physio class that will be helpful in pharmacy school than I learned in my micro class. As far as preparing for the PCAT however, I really don't know. I took the PCAT last June and I don't really remember many micro questions on it, but then again I don't really remember many physio question on it either.
Anyways, I would say Physiology would be much more helpful overall.