I just traded my 92 Miata for a 99 Boxster (and the dealer is making a killing on the sale of my Miata.... I should have sold it myself
) so I think I have a good feeling for the differences (yes I also looked at the new MX5).
The Miata is a fabulous car. Mine had 130,000 miles on it when I traded it and NEVER, EVER had a problem. It would sit under a bank of snow for months and then start as if I just drove it 5 minutes ago. Heck the original battery lasted 128,000 miles and 12 years! Get the picture? RELIABLE. It was also fun to drive. Whenever I was depressed or in a bad mood I would drive it and it put a smile on my face. One person described it as a grown-up's go-cart and thats the best description I ever heard about the car.
The Boxster, at least from my readings on this board, will not be anywhere near as reliable. BUT it has a beauty that the Miata does not. The lines (IMHO) of a Boxster are much more beautiful than those of a Miata. As far as fun to drive goes.....well this is a tough one since it almost like comparing apples and oranges. The Boxster is as fun to drive as the Miata, but it does not have that go-cart feel. To me this is good to others this is bad. You get the "performance feel" from the Boxster but it does it in a much more "elegant" and refined manner. So it boils down to "elegance" vs. kid type fun.
Also, ride in a Miata for a long trip and you will start avoiding it on long trips. The Boxster will make you yearn for more. Just imagine driving a go-cart for a few hundred miles..... it wears on you!
Rob