Ha, thanks for the comments.
I actually don't use either for commute. I live 2.5 miles from work so I actually run/walk to work....
Both cars are used just for the weekend.
I bought the spyder last summer and couldn't believe how good a driving experience it was but I really craved the front engine rear drive madness of a short wheel based car like the MC. I had the z3mc once and figured that since I had the spyder, I really should have just the coupe.
I really have not regretted the purchase both cars are like chalk and cheese. The spyder is sharp and communicative while the z4mC has a fantastic engine. z4mc is less sharp in every way but exudes great character. The car will bite at low thresholds if you are not careful and the combination of awesome engine plus short wheel based rear drive chassis and relatively quick chassis, means that you are always on your toes. I simply love it.
The spyder is just so focused that if you are man-handling the car, the passenger will feel every bit of it. The car just takes everything in its stride and if you don't fight it will reward with super high limits and impart a feeling that the chassis can take another 50 hp. This is not something I can say of the z4m but that's part of the fun to me. On the same test route, the spyder will be faster simply because you can use all of the power while the z4m needs careful metering to extract the best out of it. I love it.
These 2 cars are here to stay for me.
James_G , I wonder what your question would be if I told you that my porsche dealer tried to sell me a Cayman R to complement my spyder..........