Honestly they are both great cars, Ive had both. It honestly depends on what you want more. Do you like doing more work? Or would you want cheap parts? For cheaper parts and less work id say pick up the miata. If you like doing work and dont mind the money aspect as I do then stick with an rx7. Plus the sound of an rx7 is more beastly then the whine of a 4cyl haha.

 

Miatas are great cars and nearly indestructable. I drove my friends Miata to over 9hrs to Pensacola, FL. "Navy Guy" Topping of the oil at every gas stop. No A/C. The convertible top was torn, windshield cracked, and for some reason he thought it would be cool to paint the wheels yellow -_- Pulled hard to the right. The car literally had a straight pipe with a fart cannon muffler. Exhaust drone nearly drove me to insanity! For some strange reason, I sort of enjoyed the trip lol. Anyways, Moral of the story.... Buy a FB!

 

I own a base 90 NA Miata MX-5 (which came from the factory with no radio) with 26k miles

and

an 85 FB RX7 GS with 74k miles.

Both are completely stock and very fun cars for very different reasons.

Engine: RX7 has a much more interesting engine, even as the 12a edition, it still has some great pull and actually I enjoy the way the carb reacts in comparison to fuel injection. The Miata's engine is quick (1.6 version, not sure about 1.8) but it really requires you to bring the engine up to at least 4500 rpm to really feel like you're having fun. Also the 12a sounds way better getting up to speed while the 1.6 sounds like it is struggling. The exhaust note on the MX-5 is really nice at low rpm though. Acceleration is about the same.

Chassis: You can really tell they're both Mazdas from many of the same engineering teams, though the 5 feels much more modern.

Suspension: Easily the 5 is more surefooted, neither has a LSD.

Interior: The 7, while not really a touring car is definitely more refined in this way, the 5 is spartan it doesn't even give you any warning for leaving your lights on. Miatas have a crazy powerful fan in them, though both cars have no problems keeping me warm in winter.

Overall Impression: I love my 7 for its uniqueness and overall feel, the 5 is more of a nice day car that while it is more able to do well in race (never raced either car) I still feel like the 7 is more of my own personality.