Kind of funny to compare these two cars as the Boxster is in a higher class. You compare the Spyder with a Miata or MG TF, and the Boxster compares with the S2000 and cars in the $50K range (Z4, TT, SLK 280, etc.).
Having driven both, I can say they are just very different kinds of cars. The Boxster is a lot heavier, more solid, less tossable, and a very important point is that it's not really owner-serviceable. We have a huge advantage in that we can easily wrench on our cars, and repairs and maintenance are much cheaper all around. The Boxster does have a great motor and it delivers long, steady power all the way to redline.