I think one thing to think about is that having a PA major will likely NOT give you an advantage for getting INTO med school. That's largely dependent on your MCAT, GPA, extracurriculars, etc. You may also have trouble getting the necessary letters of recommendation if you're in a program designed solely to graduate PAs. Also, having the time to do volunteering, physician shadowing, and all those types of things is important, and being in a PA program might not be conducive to that.
Your PA coursework probably won't help you on the MCAT, since it pretty much has no clinical focus. (It's really about how well you learn the stuff in your pre-req sciences)
Whether or not having a PA degree will help you IN med school is debatable, I think. If you'd like to read about what the differences might be, look at corpsmanUP's 2nd post on this page:
http://forums.studentdoctor.net/showthread.php?t=202203&page=2
Good luck