Anyone have info on the primary differences between the 24 and 28 pin versions (aside from 4 pins) of the 261 200 059 ECU?

I know one has to know if theirs is 24 or 28 to order an appropriate performance chip...but are the ECU modules interchangeable as a whole such that if one is replacing the entire unit, it doesn't matter the number of pins? <P><a href="http://www.bmw.com" target="_blank"><IMG SRC="http://www.z3bimmer.com/signature/sig_bmw.jpg" BORDER=0 ALT="BMW.com"></a><a href="http://www.z3bimmer.com" target="_blank"><IMG SRC="http://www.z3bimmer.com/signature/sig_z3.jpg" BORDER=0 ALT="Z3Bimmer.com"></a><a href="http://www.e24Bimmer.com" target="_blank"><IMG SRC="http://www.z3bimmer.com/signature/sig_6er.jpg" BORDER=0 ALT="E24Bimmer.com"></a><IMG SRC="http://www.z3bimmer.com/signature/sig_ult.jpg" BORDER=0 ALT="Just the Ultimate Driving Machines">