The Alpina is going to be a lot quicker around town - especially here in Chicago (AWD + the extra torque are going to be very very nice). The car is also very rare.
The Alpina does however depreciate like a rock and worse, dealers are much less likely to deal on them compared to the M6GC - so pricewise, it will be hard to beat the M6.
I do however happen to know a dealer with a black black Alpina 6GC inbound if you are interested
