In the case of the e30, the M3 was the car of choice to race due in part to the high revving horsepower potential from the 4 cylinder. The fact is though that on the street, the 6 cylinder with a few mods would be a better choice. In the case of e21's, I suggest going with the six cylinder due in part to the limited amount of mods that can be done to the 4 cylinder. The 323 already has a beefier suspension and the amount of mods needed to get it to 250 to 300 horsepower is not as much. My .02