Where do you live?
If in the US like many members, does it really matter from a performance standpoint? The last time I checked all BMWs except the Ms were limited to 130mph. (Like you would drive 85+ for more than 5 minutes anyway in the US....)
Cars are my livlihood - and 50% of our cars go to the US market. It has been whispered - of the record - that that market wears its cars instead of drives them, but that it too simplified.
The US roads, by and large are wide and straight, but in comparitively poor condition. The demands are simple unless the driver is pushing the car and the legal limits. (Same applied for Japan - except the roads are almost flawless...opposite= A10/A12 LaSpezia to Nice or Futa/Raticosa...) I have spent and spend several weeks/months a year in the US, with professional drivers, mechanics, designers and engineers (like myself) - and the chance to push any of our cars, or even the two you mentioned, only exists on the track. (Northern europe too, but Germany is the last northern hold out....We still test the finished cars on public roads in Italy
)
The honest answer - buy the 6 cylinder BMW and equip it with a 6 speed and the "sport" options. If you are a label fetishist, order it without the numbers on the back - or if you need the ego boost - buy the numbers for the 8....
Besides, Porsche stopped making sports cars a long time ago.....
:2 cents: