In first sight the Porsche 924 Carrera GT and the BMW M3 (E30) seem to be difficult to compare. But when you look closer there are actually many similarities.
Both cars have been designed and built with homologation rules (FIA) in mind, so both cars are Evolution Specials. Both cars are based on a mass production everyday car, the Porsche is based on the not too popular but still well sold 924 (with Audi engine), the BMW on the very popular 3 series.
Performance of the two cars is pretty similar, despite the 7-10 years of difference in age. They accelerate in roughly 7 seconds from 0-100 km/h and reach a top speed around 240 km/h. Both cars are equipped with 4 cylinder engines. And in both cases the car manufacturers decided for more or less traditional tuning options to make the cars faster and better looking, i.e. body kits, aerodynamics improvements, engine tuning, weight saving, etc.. In both cases the result is absolutely convincing! And this is appreciated by people owning and buying these cars. Both cars hold their value much much better than their mass production sister. A well maintained M3 (E30) from 1987 to 1990 will fetch prices around Euro 15'000 to 25'000, the Porsche 924 Carrera GT typically trades between Euro 25'000 to 35'000 for examples in good condition. And, yes, both are fairly practical cars, can seat 4 people and take some luggage too. And both cars are astonishingly economical in terms of fuel consumption, at least if you believe the ECE/DIN average consumption levels.
But let's get to the differences between these cars! The Porsche 924 Carrera GT is 30 years now almost, so basically can be called an oldtimer, while the M3 still needs to wait for another 8-10 years for this "honor". While the Porsche has a turbo engine with more then 100 hp/liter, the BMW uses a high revving 4 cylinder racing oriented 4 cylinder without turbo or compressor.
The Porsche has been built 406 times and probably 90% of these cars are still around. The M3 in comparison has been produced about 17'000 times, so in terms of rarity there's quite a difference! Interestingly this doesn't impact the value too much!
If you want to buy one, then color choice will not be of great concern as the Porsche only was delivered in three colors (red, silver, black) and the M3 in only few mores (e.g. white). Condition and history are critical, as maintenance bills can become quite significant for both cars!
I have been writing about these two cars before, so if you want to read more about them, have a look here and here.