Really interesting question. It depends on what you define as power. PMs are responsible for building the product (so they ultimately have the final call on product decisions), but PMMs are tasked with getting that product out in front of the right target user. You can have the best product in the world, but if it isn't marketed, no one will know about it.
The ideal situation is that PM + PMM work together, tackling big decisions together and that nothing is done in a silo. When you have a rogue PM that makes decisions irregardless of a PMM's customer data, then you have a big problem.