Yeah, unfortunately, it's so hard to judge because a Porsche's lifecycle tends to be restricted to a certain type of owner who owns his car a certain length of time and drives and maintains it a certain way, whereas BMW's end up in the hands of people in all walks of life.
ANyway, this mileage issue has been something that has bothered me about the Boxster for some time. Although, like Brucelee said, it's so new that it's hard to compare. The ones with >150k are cars that are driven ALL THE TIME.
Anyway, BMW or Porsche -- if you maintain it right, you should definitely get a lot of mileage out of it!
You can't go wrong with either.