I have been in marketing and product org -- both places and see pros and cons for both. It all depends on the strength of the PM team and their skills.
> If PM team is strong enough to own end-to-end product responsibilities including how to build for new verticals etc, -- then PMM under marketing would well. This gives lots of advantages as highlight above by others - budget, go-to-market, campaign alignment. The normal human behavior of "being in the same tribe" :-). also because most marketing organizations are dominated by women.....
> If PM team is however not strong especially on the outbound strategy, market analysis, customer segmentation etc -- ie. they are more user design and engineering folks - then there will be a big gap. PMM reporting to product org where they own this analytical piece can be a huge help. But then it results in hand-off issues with campaign team, access to budget, clarity on owning the go-to-market channels and decisions around them. Campaign team - if in-experienced - can end up dumping / putting more work PMM -- its a complete disaster for PMM team in such a scenario. Kind of being in a lose-lose situation. I have seen this as well.
So my idea recommendation is that PM, PMM and Marketing leader - understand what kind of skills is needed for a end-to-end successful growth of the product and marketing efforts. You could potentially have specific PMM leader/person in or aligned to the PM organization to fill the gap in the PM org, and other PMMs focused on the outbound. Basically someone has to do the strategic effort, else marketing will be quite wasteful and there will be big gaps in sales enablement as well.