Well the mx5 will need a cambelt replacement at 40k miles.
Indeed.
I have a '95 Miata which is driven vigorously a lot of the time.
According to the owners manual the cambelt change for US spec cars is required at 60,000 miles (or 90,000 miles for California cars), so the 40K thing is a fallacy.
I did actually change the cambelt in mine at 60,000 miles just to be on the safe side, it's a much cheaper option than replacing the engine if the cambelt fails when it's getting closer to 90K! I presume it's because the CA cars are less likely to have been exposed to lots of cold starts than elsewhere in the country.
The Miata is a very easy car for an inexperienced driver to drive at 10/10ths and be able to handle. As
niky suggests it's not likely to overpower itself (or you for that matter) and is hellishly easy to catch if it does get sideways or out of shape.
I have an old 911 too, and while it's fun (a lot of fun) to drive, the Miata is easier to trust when pushed hard. The 911, well everyone knows it's quirks, and I'm loathed to push it to it's absolute limits lest I find out the ass-first-into-a-ditch way whether they're true or not!!!
As far as size goes, there's a pretty simple modification you can make to the seat rails that'll make even an NA Miata relatively comfortable for a 6'4" and relatively portly male (there's a guy fitting this description in the Miata club we belonged to) and besides, there's an unwritten law that says unless it's -30 degrees outside, raining like when Noah built the Ark or you're planning on driving through Tornado Alley, then a convertible top should be firmly anchored in it's proper position, i.e. DOWN, anyway!
I've never driven an S2000 though so I can't comment on how easy they are to push nor their relative space.
and a little problem...that the S2000 has lasted through two generations of Miata, and still seems like a fresh car.
Well, I suppose technically yes, through the NB and NC generations, however, the Miata being originally released in 1989 has a 10 year headstart on the S2000, and is still selling...