With the M3, Z8 Alpina, and the 4.8 X5, you certainly have the expertise to make that judgment (although the M3 is easier to drive). What fun to get up and have to decide between the M3 and Z8.
I do know that with the Z8 sitting in the garage, I wouldn't feel so bad about having the M3 as a daily driver. It really is the best of both worlds. I can only imagine what kind of attention the Z8 brings, because I do know how much attention the M3 draws. What fun!....