Mike: here's my $0.42:
As mentioned above, the S2K needs to be rung out to high revs in order to maximize the VTEC motor. As such, be leary of cars that may have been "ridden hard, and put away wet." (IE: stay clear of younger kids who are quick to tell you how fast the car can go, and the like...) From what I've heard and read, the S2K really lacks low end grunt - if you like to squirt away from stoplights, or just in general like the feeling of low end torque, then this car isn't for you. That said, the S2K is a very nice modern car -- and in my area, it is far less common than the other 2 seat roadsters.
The Boxster/Boxster S is a competent car -- if you are concerned about durability, opt for the Boxster S -- I think the regular (2.5l and 2.7l 986's) had more RMS issues than the 986S did. Plus you get better brakes and suspension with the S. If I were in your shoes, I'd get the Boxster over the Honda -- I believe that German engineering really produces the best cars these days...
As for the Miata -- for $20k, you may be able to pick up a nice Mazdaspeed Miata -- these are the turbocharged cars. (Just verify that it is a factory turbo, and not a home-made 'fake Mazdaspeed Miata.') My neighbor has one - and it is a fun little car. However, if you are on the tall side, you may be a little cramped inside that car. Plus it is a bit more primitive than the S2K and 986's.
You can find a first-gen Mercedes SLK for around 20k. I've seen SLK32 AMG's going for under $20k. Very comfortable car - and the retractable hardtop keeps the bad weather out. And the AMG version is quite a little sleeper with over 300hp!
Another car worth looking into is the Audi TT 'vert. The Quattro version will give you sure-footedness you can't get in a 2WD car. (And there's that German engineering again!)
Personally, I'm not a big fan of the BMW Z3 / Z4 roadster's looks, but they can also be worth a look.
The Pontiac Solstice / Saturn Sky are fun little cars, but in a few year, I suspect parts may be hard to come by.
It sounds like you aren't into the boulevard cruisers like the Mercedes SL500, Lexus SC, or BMW 6-series 'vert. Plus they are a little beyond your price range. A Vette 'Vert is also doesn't sound like your style.
I think that just about covers most (if not all) of the modern two-seat roadsters out there within your price range.
My top two choices would be a Boxster or SLK AMG. But I am a bit biased!
-Z-man.