First question:
Do you like Front wheel drive?
Do you like unique sounds and handling?
Or do you like a car that handles easily?

Then, what fits the competition where you are?

The Miata has it's advantages:
Mazda support (huge), dream car handling (beginer friendly) Pretty cheap to run, lots of competition at all levels, (good for learning racecraft). And you can convert to ITA, Prod, etc., If you do well, you can go national, or to big events.

And it's disadvantages: It s gehy. (sorry, it is! ) it's often dented, and you'll be stepping into a "bad rep" situation right away. (there are those that think SM drivers are crashfests waiting to happen. Whether that is true is hugely debatable, but with so many Miatas out there, there are bound to be incidents), and it's a tight fit for taller or wider drivers. Some find Spec racing too limiting, and like the variety that mixed marque racing offers.

The RX-7
Advantages: Rear wheel drive, handles very well if set up right, Hard to hurt the motor, revs forever, (good for beginner), Mazda support, and super cheap buy in. Runs in either ITA or IT7 (Both if they are separate run groups), and ITA is a popular class.
Disadvantages: RWD..if you're a FWD kinda guy. It's getting old, and some parts are tough to find. If you are afraid if different engines, the rotary will alarm you. Scary handling if it's set up wrong.

Rabbit- Advantages: Cheap to buy in, lots of parts, ITB is a cool class. FWD,(Some say easier to drive)
Disadvantages. Probably stuck in ITB, (unless the chassis is also in ITA with a differnt motor, I think that's the case with later versions of the Golf), Certain version are prone to break hubs.

Hope that helps more than it confuses!