I have a 2000 M5 and a 1995 C2 Track Car. As such they are very difficult to compare and contrast. However, prior to the 993 I had a '97 M3 that I used primarily as a track vehicle, although it was not nearly as modified as the 993.
I really enjoyed the M3 it was a blast to drive and rarely would let the driver get into any real trouble, if you start a relatively stock M3 doing anything but understeering you are in for a really big motor vehicle experience!
I love the 993 on the track much more "driver involvement" to run a the limit. This is especially true for my vehicle given the suspension has been tuned to dial out understeer.
I must say that the M3 was quite a bit more reliable than the 993 never breaking a single part in over 8000 track miles. The 993 has broken several suspension pieces one during the middle of a contested corner in a PCA practice session (
). While it is unfair to judge the 993 because of the extensive modification, I have been disappointed with its reliablity factor in comparison to the more lightly modified M3.
The M3 ate through brakes at a frightenly quick pace on the track despite euro rotors and some massive air ducting. The Porsche (big reds) does not have quite the appetite for brakes even though the braking temperatures reach some amazing points (both cars ran Pagid Orange).
All in all, I'd say that Porshce might be able to learn a little bit about reliability from BMW and BMW might be able to learn a little bit driver involvement and brakes from Porsche. (note this seems to been recently reversed with the intro of the somewhat bland driving 996 and the rather exciting e46).