That's a fairly 'meaty' discussion. You'll probably get some good info here
but I'm sure you can read countless reviews / opinions on miata.net.
Probably worth the time.

Good luck,
Chris
99BBB

Here's a summary of NA (89-97) top replacement that I hope will help you...
There are many replacement tops available and there are considerations about
appearance, cost, and quality.
They can be purchased in the original relatively maintenance free vinyl or
the more expensive and fancy looking cloth (which requires waterproofing
from time to time and is less forgiving about clean up with contact to
unusual things like tree sap).
The top can be exactly like your original, made from two pieces, one for the
top and one for the window, or it can come in one piece.
The top can be of the non-zipped or zipped type and can have a plastic or
glass window.
To install a non-zipped top on a NA (89-97) requires some drilling into the
second and third bows for the installation of some straps that keep the top
positioned correctly when closed.  In this configuration the top it not
attached to the third box and you have to reach inside to pinch the third
box toward the second when opening the top (and vice versa when closing the
top).  Not a big deal at all, but different than what you are used to.
It is worth buying a new rain rail (gutter that runs along the rear edge of
the top) if you have no idea of the condition of the current one.  An old
rain rail will likely be riddled with cracks that cause leaks and to patch
it with silicon is time consuming and may not work.
Also, there are cables that run along the side of the top that might need to
replacement.  These are called top tension cables.  If the top it loose
around the edges against the windows, your cable may have snapped and need
replacement.  If the top seems fine then you probably do not have to replace
these.
There are many dealers for tops and the short list is:
Robbins (available from miatamania.com and others)
Duettomotors.com (sells robins and their own) - offer good installation
instructions with purchase)
Kwiktops.com (sells robbins and their own)
For what it is worth, I highly recommend getting a glass window.  Being able
to see is the best safety feature you have.  Since I like the original oem
style two piece look better than the one piece, and I like the cloth better
than the vinyl, that is what I got.  But, if on a budget, or for a car that
simply isn't long for this world, it might make more sense to get a
inexpensive vinyl top with plastic window for as little as $200.

ps. I have seen that some of the one piece glass windowed tops do not sit as
low as the stock top when opened.  This means that with those models you can
not use the original boot to cover the open top.  The animated one piece top
on duetto.com is a perfect example of this, showing the open top sitting
above the beltline when opened.  I am not certain, but I think that this was
an issue with earlier robbin tops and they have since sorted out the
problem - worth asking robbins about it before ordering or you may be
looking at getting a new boot too.

Yeah!

Having had a '93 with the zippered plastic window and a '99 with the
fixed glass window, I can tell you that the glass one is a lot easier to
see through than an 11-year-old scratched plastic one, and it only takes
five seconds to drop the top on my '99 compared with 20-30 seconds for
the zipper top.  (The reason it used to take so long is because I'd
always a.) lay a towel on the rear deck, b.) unzip the window, c.) lay
another towel over the window, and d.) drop the top.)

But if I ever have to replace my '99's top I'll be looking for a top
with an unzippable plastic window anyway.  They're cheaper, the window
is bigger, and when they're less than eight or nine years old (and the
'93 spent its entire life under the plastic-eating Florida sun) they're
pretty much equally transparent.  The big advantage to me is that I
could drive around with the top up and the rear window down, which is
real nice either in the hottest part of the Florida summer with the sun
beating down on my balding head, or also during our interminable monsoon
season.  Also if you absent-mindedly lock your keys in the car, all you
have to do is slip your hand between the top and the window, push on the
slider from behind and unzip the window, and presto you're in.

If I lived some place where it snows, I'd probably go for the glass
window, though, as it wouldn't crack in the cold and it has an electric
heater built into it.

Yours jp

coach, I bought a '90 last year w/ a new zippered glass window. Love
it! I believe the brand is E-Z.

Chip L