keep in mind that a 6-speed gearbox isn't necessarily better than a 5-speed. The gearing itself is what matters. Also, what your intentions are...ie. do you want good 0-60 or 1/4 mile accleration, higher top speed, etc.
I believe the Miata anniversary edition came with a 6-speed and the 0-60 figures were marginally SLOWER than those put up by the past year's model that had the 5-speed.
just a thought...whether this would be true or not on our car, who knows!