An unlikely comparison you might say? Absolutely nothing in common you might also say? Well while you are right, I enjoy comparing cars that really have nothing in common except for a key factor; FUN. Both cars excel at fun for different reasons and so that is why I think they can be put head to head. I have just completed week 2 of ownership with the STi and have had the Miata for 3 years as of now. So my opinion may not be fully developed just yet but consider this my initial thoughts.
Some notable technical differences are: RWD vs AWD, 142hp/125lbft vs 300hp/300lbft, $24k MSRP vs $33k MSRP, 2400lbs vs 3400 lbs, and obviously 2 seat roadster vs 5 seat sedan. Now the other important factor is purchase price used, I picked up the Miata for $5k with 125k miles on it and the STi for $13.5k with 136k miles on it. The Miata currently sits just over 200k, has been modified a bit versus the STi that is totally stock (other than the spoiler delete). I can say right off the bat that stock vs stock, the STi is INFINITELY more fun and better in every way. Like its nearly not even a comparison. People seem to like stock miatas but no way would I ever compare that to anything!
Now while I have not put that many miles on the STi yet, I have definitely been driving it spiritedly enough to determine how much I enjoy the car. This past weekend I got around to putting new tires on the car and spent three days of commuting the car in a very spirited manner. Mostly via Mullholland Highway, Encinal Canyon, and Decker Road. These roads I have also driven in the same manner in both directions in the Miata. I say both directions because its a distinct difference going uphill most of the way (heading home) versus downhill (in the morning).
Going uphill, its a laughable comparison. The Miata is powered by a boat anchor and the Subaru is a rocketship. Surprisingly though, it can still be just as fun in both even then! The Miata with the loud exhaust, open top, racing seat and harness, and tendency to kick the rear end out playfully in tight turns if I gas it makes the car feel/sound more like a race car. It honestly grips less than the Subaru and is obscenely slower but the way it body rolls around the turns and makes you experience every ounce of grip it does have is just magic. However, the Subaru like I said blasts up the mountain and seems to have infinite grip around every turn. It will get a little bit pushy at the limit but it seems to be very gradual and communicative about this. Any time I can get on the gas though, it is just INFINITE GIGGLES. Every time, the fun pedal just makes the car teleport to wherever I point it. This does not get old although it certainly burns WAYYYY more gas in doing so.
Downhill/mostly level though is another story. While of course the STi is still way faster, I dont immediately notice it nor do I miss the speed while in the Miata. The STi will still corner better and maintain a higher rate of speed. Plus I can mostly leave the car in 4th/5th gear around most of the less tight turns whereas the Miata is in 3rd. But anyways, the Miata comes back to shine here during any of the lower speed turns, which there are many on these roads. This is where even with less grip, I can get around the turns MUCH closer to the limit so I end up realistically not going much slower. I have to maintain momentum more but I am more used to the car so its not that challenging to do so. I havent timed the difference between the cars since it is street driving but I would think it would come out relatively equal going downhill.
Fun driving aside, is it really possible to compare the two cars? No not really. The Subaru has regular car seats and the Miata has a knockoff Bride bucket. The Subaru is on normal suspension that doesnt want to murder my insides and the Miata is on coilovers. The Subaru has an actual trunk, a back seat, and a cabin that can be closed from the outside world, and the Miata is well, a Miata! So really its clear to me that while both cars are fun, the Miata is of course not a very practical car and that Subaru can be both fun and practical.
But at the same time, the Miata can easily achieve 30mpg and I would be hard pressed to get more than 20mpg in the STi. Sure I could probably learn to stay out of boost in the STi and achieve mid 20s on the highway but in the Miata, you can drive it like you stole it and still get 28mpg. And I have learned how to maximize the cargo space in/on a Miata to make it work for any variety of needs. Plus the STi is certainly going to be more expensive/difficult to maintain when things go wrong on it eventually. But if I keep it stock, I should not have too many problems (famous last words).
Overall, I thoroughly enjoy both cars so far and am looking forward to eventually trying the STi at an autocross as well as driving it in some snow with snow tires. That will get me a fuller picture on how the cars compare and likely make me appreciate both cars even more. It really is a good combo to own both of these since they dynamically oppose each other while still giving me plenty of the same fun.