Thanks for sharing your impressions of both the Boxster S and the Miata.
It's always fun to see comparisons between two sports cars I greatly enjoy.
Right now I'm on my third Miata (an '06 Sport) and have been a Boxster S owner for less than a week. Thus far I haven't been able to do an exhaustive comparison of the Boxster and the Miata but can share some initial impressions.
In spite of the fact that the Miata is front engine/rear drive and the Boxster is midengine/rear drive, the cars are otherwise similar in many ways. Especially when comparing the current-generation (NC) Miata to the Boxster, the cars' dimensions are similar. Both are fun to drive even at low speeds and appear to be well-built. But given the fact that the Boxster is much more powerful and solid and is quite a bit heavier than the Miata, I'd go so far as to describe the Boxster as a Miata on steroids. It's at higher speeds and on the open road where the Boxster seems to really shine. I haven't had a chance to autox the Boxster or drive it on a road course yet, but I imagine it will perform very well.
I'm glad I'm able to own both cars because I don't know which I'd pick if I had to choose between the two of them.