Okay, I'm back--back from SEMA and back from some running around here at home. Sorry for the delay.
So, the original question: Having said all of that, and seeing that you own both, which is the car you'd have trouble parting with and why?
If you held a gun at my head and demanded the keys to either the Porsche or the Miata, I'd have to hand over the Miata.
I have been thinking about this for a while, but then realized the answer was staring me in the face: I keep three cars (Miata, Porsche and my wife's Civic Si) crammed into a two-car garage. The Miata is parked sideways in the back corner; the Porsche can come and go as it pleases. The poor Subaru lives in the driveway.
I'm not saying that one is a better track car or autocrosser than the other, but as far as the driving/ownership experience, the Porsche is just a sliver better. The Porsche can just do a bit more--it's an iconic '60s design that can easily go cross-country. I can also autocross it, track it, and concours it. My guitar fits nicely on the rear package shelf. My wife and I can easily go away for a long weekend in it. Plus, at least here, the PCA adds a little something--events, social activities, etc. Not that it's all about dollars, but the Porsche is also worth more and has much more chance for appreciation. (And conversely, the Porsche also cost more to buy.)
The big thing, though, is the total experience: the sound of the exhaust, the smell of what can best be described as "hot oil," that amazing steering, and the view over those fenders. I have to admit, the Porsche is the first car I have owned that I'll just look at and tell myself, "Wow, I own that."
Of course, this doesn't mean that after 14 years of ownership I'm looking to sell my Miata. It's also an amazing car to drive, especially with the top down.