I'd go with a Boxster S. I just read some reviews of the Boxster S compared to the new Z4 roadster. I'll steal a quote:
"If the Z4 M roadster were the only car we drove on this magnificent road, we'd walk away perfectly happy-thrilled, even-with its performance. But then we trade the BMW for the Porsche, and the Boxster demonstrates once again that it is the more natural performer. It seems custom-built for this road as it shrugs its shoulders at hairpin after hairpin. Its brakes never protest the harsh treatment we subject them to, and its wailing flat-six provides plenty of power. On this road, the Porsche makes us feel like we couldn't possibly cover ground faster or more easily in any other car."
They also like the Caymen S over the more powered Z4 coupe. Ilike and own a BMW, but the Boxster is a very good car and I am always looking for a good deal on a used S.
For more the complete article.
http://www.automobilemag.com/reviews/convertibles/0605_2007_bmw_z4_2006_porsche_boxster_s/index.html