They are all different sizes and for different purposes..what do you normally carry?
The PM has a chain that has smaller links compared to BB and MM (which are the same size links). It all depends on your preference, what you carry, and maybe preference on color.
I have the PM which I think is a great size. I don't over stuff it as not to affect the embossing/stretch out the leather. I only have like a slim purse wallet, phone, key pouch, hand lotion, sanitizer, lip gloss.
Unfortunately due to price increases, the BB is now more than what I originally paid for the PM. Definitely go to the store to see the size differences. The leather is sooo soft though - I love it.