As a couple have said, it's all in the importance of a top-down experience.
The Boxster is a purpose-built convertible. It has the structural rigidity, balance, and design expected of a convertible. The 911 is a purpose-built hardtop sports car. It has all the previously stated attributes associated with a coupe. So I would base my decision on where you want to comprimise your driving experience: straight line acceleration, or top-down experience! If you remove those two factors, the cars are a toss-up.
Oh, don't forget, the new Boxster comes with a warranty. Also, to be fair, I was not in a position financially to buy a late-model 996 when I bought my Boxster; I was comparing it more to a 993, M-Roadster, or M3 convt. So my opinion may be biased.
Bottom line, though is that you can't go wrong!
