I've been following this discussion with interest. My experiences with projects is limited to the one I did for three years as the JAG's PD on a project to build a records, knowledge and case management system on its own architecture with JAG/ADM(IM). The PD - my predecessor, me and my replacement [when I timed out at 60] - were all legal officers from JAG. The PM and Fin guy were civilians from ADM(IM) and the project engineer was a CELE officer from the Sigs branch. We had a number of military and civilian staff on both sides.
In effect, I developed the system requirements with my business analyst and a number of military and civilian staff - effectively defining the SOR - while the PM and engineer with lots of fin input worked out the practical solutions to the end state that the office of the JAG was looking for.
There was no seniority. It was a collaborative process (one of the reasons why I was brought in was that my predecessor - a JAG major - wasn't good at collaborating (after about two years on the project there was still no SOR and the project was still conceptual in his head). We met weekly to discuss progress and then went off to work on our respective responsibilities. Once we got the SOR out, my role was to monitor the PM and her team's progress on engineering and acquisition while I moved on to developing the business transformation plan and its various components for the office of the JAG.
I find the idea of who is senior a bit strange because neither the PM nor the PD side have all the answers by which to make and push a unilateral decision. It's collaboration and give and take where requirements become technically or financially impractical or impossible and in trusting each side of the team and giving them the space to work out the best practical solutions. In our case there were five principle folks, the PM, PD, Fin guy, business analyst and engineer. When they don't work well together one needs to take action so that they do. The JAG put me in to replace my predecessor when things weren't working out and about 8 months in I fired the business analyst and lucked in by finding a Cracher Jack replacement. That, IMHO is what matters. Creating and maintaining a team that can work together and having a PD and PM that each knows their side of the job well.
$0.02