I am torn, I like the NB series, as more affordable, and the proportions look slightly better, especially with the skirts. But I like the NC series with the hard top, to drive all year, and for better sound insulation when the top is up. The leather in the NC series is better as well...
The MX-5 has very little competition, and the Z4 isn't one of them. Nor are the pony convertibles, or the 370z. I would say the Z3 was a little closer to the MX-5 in terms of hp, pricing, and interior quality. I liked the cleaner lines of the Z3 compared to the Z4. But on the other hand, maybe a Z8 would be great, if we had the money!