I would go with 195 width. It is a nice width for a Miata and you will use the extra grip. With a 1.8, the gearing advantage of the 185 is not as important.
BTW, Treadzone has some nice prices if you plan to shave them. You will probably have to call them instead of using their web site, though. If you plan to get full-tread, then Tire Rack is probably just as good.