I am a current owner of a Miata and am looking into upgrading to a Boxster. Let me tell you, every time I drive it, I ask myself do I really need a Boxster. It is so fun, light, handles great and doesn't feel slow at all. I can reliably add a supercharger for a solid 220hp on a 2300 lbs car! But in the end I think I want a more upscale, refined, and more powerful version of the Miata which is a Boxster in my opinion. The Miata is just a bit too small at times and a bit too rough and no wow factor from onlookers

The new Miata is definitely very tempting though at a fraction of the cost of the Boxster. About $18k-$22k brand new!
http://www.miata.net/news/nc_images/nc04.jpg
http://www.miata.net/news/nc.html