This is the question of the ages. In fact, it is an interview question when I interview prospective project mangers. (not that we have been doing much hiring in '03). If you think you are a PM, you can prove it by taking about the differences in these two tools.
Primavera is very entrenched in the construction and EPC industries, although the recent upheaval in the EPC business will soon lay waste to any entrenched ideas and tools. I have not been able to sit an engineer down and get him to pick up Primavera and run with it, despite (or because of) its limitless flexibility. Professional schedulers an dsome PM's have no problem with it, and may have taken formal classes in Primavera.
On the other hand, even senior engineers (the slowest learners) can pick up MS project and document what their schedules look like. Soon they can baseline a project and track changes, as well as assign blame for delays (they love this) using the tracking Gant and notes features.
My vote goes to MS Project, not becuase of power or features, but due to its sucess in getting used on actual projects.