I’ve spent last few days on PMI certification preparation course. I didn’t take the exam yet (I want to get through Rita Mulcahy’s Prep Book first), but I guess I can already make some statements about: 

  1. Comparison between PMP and PRINCE2
  2. PMP knowledge being applicable on my projects
About the comparison:
PRINCE2 is far more high level - it helps to structure the project and set-up a proper project hierarchy. Its approach to project’s flow is very traditional, but generic enough to make it adaptable. What is more important, PRINCE2 distributes project life-cycle into parallel dimensions - project management and technical products creation. Both of those may have a different phase split and PRINCE2 cares only about project management perspective.
The main difference is that PRINCE2 is a methodology and PMP is just a collection of techniques - best practices that may (and usually are) be applicable on your projects. So, simply speaking: PRINCE2 answers the questions “WHO?”, “WHAT?”, “WHEN?” and PMP answers the question “HOW?”.
About the PMP being applicable:
If you’re already familiar with any waterfall-based methodology (pretty much every big company has its own fork or adapts something), you won’t be surprised with anything in PMI’s approach. Quite the contrary, it will appear very similar to what you already know. You can treat it as a good refreshment of project management detailed techniques - none “translation” (PMI to your stuff) effort will be needed.
Now, that’s just one thing to do - an exam. I’ve heard it’s actually harder than PRINCE2 Practicioner. I’ll see about that soon.