From an E30 perspective I would say there is very little in it in the first place unless you start comparing the different extremes of the 4 pot-6 pot engine range e.g. M40 1.8 against a M20B25.
In between e.g. say an M40/M42 against an M20B20, there is actually very little difference in terms of accerleration and fuel consumption other than a smoother engine and bigger road tax/insurance . In fact in some critical departments the 'lower range' E30 6 pots start to lose out against their 4 pots cousins. In this case the 4 pots are perhaps 'better balanced' cars.
The M20B23s are certainly a step up from the 4 pots, offering better power/torque etc. but still not that much more..... (they are also older engines) ....it is only when you start comparing the M40/42s with the M20B25s does the 6 pot BMW start to make sense. The range of upgrades available at this end would certainly end any argument over who's better .... a stroked 2.5 could be made to achieve in excess of 210bhp with the right mods.
From a larger perspective, the M50, M52, S50, S52 series onwards are more the kind of engines BMW is reknowned for or should be making. They represent some of the best in 6 pot design IMHO and best represent what BMW is all about. Their 4 pot cousins are simply a reflection of the need to reach a wider market. Bear in mind that the 4 pots at this level represent some of the best designs available and, correct me if i am wrong, are among the most power standard, N/A engines available on the market today.
The future is diesel I think.....
