Which car will be better?

The Miata is better!

The 944 brand new only had 150 hp, the EFI was pretty primitive when compared to even a 1990 Miata. It is not a true Porsche, it is more Mitsubishi and Audi, so reliability wise a big risk. Think about this, it only has 80K miles on it. To me that says it is a Garage Queen.

If you plan on up-grading any way, the after market support is much better for the Miata.

But a point on up-grading-

Check out spec Miata racing in your area (and autocross rules). Do not build a car that puts you into a bracket where your driving skills keep you from being competitive. It would also be sad if you showed up a Watkins Glenn for a race and could not pass tech because you had done to many up-grades

Some thing to consider:

Grassroots Motorsports has done several "Project Miata's", and many of these projects can be reviewed on line.

Mazda, Why ? More reliable, Parts are cheaper and easy to get, Mods are plentiful and cheap.

Can you imagine the cost to replace Say a clutch on the Porsche? Getting your hand on a new panel? Parts are dear of the German Motor way muncher . Porsche is hands down a better car but it will cost you alot more to keep it competitive on the track

I would go with the 944 in this one, the Miata has always been considered a "very fun" car to drive but slow as hell.

Instead of guessing, you could use a car performance simulator[1] to virtually have both cars, even virtually do mods there and see which one performs the best