The 87-91 is supposed to be the better, but the updating and backdating rules almost negate any advantages of certain years.

2 or 4 door? Not sure that it really matters, the e36's are better in 4 door (stiffer) and the e30 2 doors are better because you can make minimum weight more easily.

Personally I like 4 door racecars, as you can more easily use the back of the car for storage and transportation, but I'm cheap and have very little of both.