You guys need to learn more about rotaries before you speak false hoods. I know of several RX-7's with over 200,000 miles on the stock motor with NO rebuilds... NA rotaries are virtually indestructable. It is the turbo charged rotaries that have reliability problems, and then, only when modded poorly or poorly maintained. My Turbo II had 118,xxx miles on the original motor when it finally let go, and that was with me running additional boost and only modest fuel mods. I have a friend with a T04 car at 108,xxx on the original motor.
To the question at hand... Are you a big guy or a little one? Silly question? Maybe, but very relevant. Miata's don't possess tilt steering, are very narrow, and have little leg room. I am 6'5". While I can fit in a Miata, I wouldn't want to spend extended travel time in one. That being said, I have spent the better part of this year autocrossing a turboed 96 Miata. It is lighter and will out handle the FC convertible, and if you decide to add a turbo, it will be quite a bit faster both straightline and on course. It is an incredible amount of fun.
If the Miata in question is a 90-93, then I would probably pass on it and go with the FC. Free is free, and if you don't take it, at least get me the hookup.

I am biased against the early Miata's because of smaller 1.6L motor, which in earlier years (90-91) had weak crank snout problems; the smaller differential, which is not as capable as the Torsens found in the later cars and not as upgradeable either; and the smaller brakes, that are sufficient, but not as desirable to me. I can't help but look at the car as a future turbo car, so I think of the things I would want so that I could put that power down. If the Miata in question is 94-97, then I start salivating a bit more. Stock for Stock, it is more on par with the power level of the FC and will handily dispatch it in the twisties. If it is a C package car (tan leather/tan top) then it will already have the torsen diff.
I need more info on the two cars you are looking at to really tell you what I would end up choosing and why. What are the model years of the two cars? Description of options and conditions. Don't 2nd string the FC just yet because it is a rotary. It may still end up being the better choice depending on the two cars in question and what you want to do with the car in the future.