I tested both. The Boxster is really dated (looks wise). The Z is more of a roadster where the Porsche is a convertible... imo.
It really came down to personal choice at the end of the day. I preferred a car that was better looking, new, and came better equipped (for the money). The Boxster I wanted to get was getting up there in price and for that I would have much rather paid a bit more and jumped into a base 911 - which is unreasonable for me right now. I also wanted the hard top over the soft top.
There were also a practical consideration, BMW offers free maintenance. This is a major selling point, especially when the Porsche rep was telling me that they have to take part of the engine apart for oil changes. Doing the math made no sense.
Overall, I felt that the Z offers and incredible balance for performance, price, practicality, and usability.
Don't get me wrong the Boxster is a great car (and I'll get flamed for this), it will outperform the Z but I would give that up. At the speeds most of us drive these cars, you'll hardly notice the difference.
Chief Cat Herder