A quick correction, the Alfa Spider/GTV bolt pattern is 4x104, which is close enough to the common Ford bolt pattern that you can use them on the car. If you see pictures of my car at the challenge, it has cheap 'stang wheels.
Parts... Has anyone ever priced Miata repair stuff? It's actually as, or more, expensive than Alfa parts. How would I know? I've been driving Miatas since 97, and have a '99 with 160k miles on it- broke the alternator and have gone though a catalyst. With the timing belt change every 60k miles, it's not a cheap engine to keep up on. You can do the work yourself, but I'd much rather work on my Alfa toys.
Reliability- Alfas reflect their owners. If you keep track of issues quickly, nothing becomes a problem. I have a SPICA pump on my '73 (see my sig) and I had it "looked" at a few years ago after 10 years of abuse- barely any adjustment needed. Electrical? All the problems on my car are more to do with the previous owner tapping into something for the radio. But when I restore the car, I'll replace the wire, since it's getting pretty old and crusty- it IS almost 40 years old, for crying out loud.
The other "weakness" in the Alfa is the head gasket- of which I've never had a problem with, either, even with mainly track and autocross use in my GTV. You'll note that at the 2004 challenge, my Spider took FTD- boosted to 14psi, running stock (but cut) pistons, and an OEM head gasket. Oh, and revving to 8000 rpm, well beyond the 6000 rpm redline. It's all about how it's put together.
IMHO, the difference between the Fiat and the Alfa is a matter of taste. I know dwight is a Fiat fan, since the last time I saw him was at an Alfa Romeo Owners Club Autocross where we hosted the Fiat Convention Autocross....
But.
The Miata is a far superior car- better handling, better starting, better driving.
Remember, I own both a Miata and a GTV, but I've never, ever raced my Miata- it's my commuter car. The GTV is the one that puts the smile on our faces when we drive it.
(oh, and if you think Miatas are not prone to rust, check the Miata rocker rust thread.... they rust, too)
The real question- what is your goal? Daily driver or emotional attachemnt?
In terms of which one to get- I'd get a '74. IMHO, SPICA rocks- starts first time, every time, even after 6-8 months of sitting out the winter.