I love Miata's. I have owned 2 different 99' models. The one I have now, I have 150,000 miles on it with original clutch. I have only replaced the alternator, water pump and timing belt on it.
Don't get one with leather seats, they are hotter in the summer and colder in the winter. Plus they have very little bolstering to keep you in the seat. And don't bother with a Mazdaspeed miata, you will pay a near $8000 premium for it in the used marketplace, and for a third of that you can get a much nice supercharge kit.
And frankly, any time you are under 6000rpm in an s2000 it will be slower than a Miata. Then there is the price. While a new s2000 is competitive with a Miata, ie slightly more expensive for a slightly faster car. However, good for a new s2000 and bad for a used s2000 is that they DON'T depreciate quickly in price. So while new the difference is $32000 and $25000, used it is $20000 and $10000. Great if you are buying a new car you plan to sell later, but awful if you are buying used.
Martin
PS the factory 15" wheels are already very light, but don't bother with 16" rims because with the super-stiff sidewalls you get less traction.