The only way you're going to know what's best for you is to drive a few of each. The E46 M3 is an excellent car with plenty of virtues (I thought at one point of buying one myself), but it's a totally different driving experience from a Boxster.
Focus more on condition and service records than mileage. I'd take a 60k mile car in nice condition that's been well cared for and has service recs long before a 30k car in so-so condition that's been through a bunch of hands and has no service recs. Buying a later model, higher mileage car from an anal retentive owner (i.e. many people on this list) can be an excellent value.
Remember, it's a buyers' market. Drive lots of cars and be picky. And when you think you've found "the one," spend the money for a pre-purchase inspection (PPI) by someone who knows Boxsters inside and out. That's money very well spent.
Read this site from top to bottom for excellent buying advice:
http://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages