Those are two very different venues. PB is pushing all-inclusive, but on slow port days may offer a pay as you go option. It is very large, with great facilities, a HUGE pool, water toys and a sandy beach. Food and drinks there are average to above average for a beach club (love their taco truck!).
MB is much smaller, no admission fee, strictly pay as you go, no sandy beach, no pool and no water toys. Good snorkeling from the shore, rental gear if you need it, and the food/drinks quality is about even with PB.
Which to choose sort of depends upon what you are looking for. If you can find an old review from Mitsugirly, she did both locations in one day (morning at MD, afternoon at PB).
Have a great cruise!