Body style NA
90-93 have a 1.6L, they are a tad lighter and have smaller brakes
94-95 have a 1.8L and bigger brakes and a bigger rear-end. They started adding more body bracing. Pre OBDII
96-97 still 1.8L the most bracing of the early pop-up headlight cars. Also the heaviest. Many/Most of the '96s have LSD
Body Style NB
'99 1.8L, but more power (10hp?). New interior, and I've read, lighter than the '97 models.
blah, blah, blah...
How much are you looking to spend. I love my '97, and my wife has a '99. My '97 has more footroom than my wife's '99.
For lots of info, check out
www.miata.net
There's lots of good info in the FAQ,
http://www.miata.net/faq/
The forums are also pretty good, and many of the folks there are as smart and as serious about their miata's as we are here. There are some kids, but it's nothing like a civic board.
http://forum.miata.net/vb/
I was originally shooting for a mid NA car, something '94-95, but I test drove everything from '90-97. Lots of folks say that you can find really nice cheap versions of the early cars. You hear about the cars that are mint for $2500. I think that's for folks that have a lot of time on their hands. I'm not that patient. I ended up going newer and newer. The last two cars that I looked at were a '96 and a '97. The '96 had LSD and was a nice dark blue with 56k miles. The '97 was black, in really nice shape, 45k miles, and came with the optional hardtop which is a pricey thing to buy usually running $1000-$1500. I went with the '97 for the lower miles and the hardtop. I eventually pulled the hardtop and put it on my wifes car.
Stock these cars have a really soft suspension. They still handle really well because their camber curve was designed to work with lots of roll. I put stiffer height adjustable springs sleeves on over Bilstein shocks, and I'm much happier with the ride.
The seats aren't very "sporty" after coming from a 911 seat. You can carve some of the foam out of the middle of the seat back and bottom and make them fit snuggly and also get you a bit lower in the car (I'm 6'2" and fit, but fit even better now that I'm 2" lower).
The engines are based on a turbocharged block, so the bottom ends are pretty sturdy. The 4 valve dual cam heads were designed specifically for the miata. The trannies are considered some of the best ever. The early rear ends will break if subjected to punishment. The later rears are bigger and better.