At the end of the day, I find that there are only ever two problems:

* Building the right thing

* Building the thing right

At every company, one of these two questions will be harder than the other and that has a big determination on the relative power/impact of the PM/EM. You have companies like Github for example where the early days customer empathy came almost for free so it was a very EM driven culture. And you have very emotionally designed products like Instagram where PM pre-eminence came from the top.

Every PM/EM relationship is divvying these two tasks with a bias of BTRT going to the PM and BTTR going to the EM but it's never a clean dividing line between the two.

In general, just do what makes sense for the team and know each other's strengths and weaknesses and especially where there are areas of mutual weakness as an area to be careful and invest more effort in.