fwiw, I did pretty much the same (with two P-cars for the weekends) but instead I have an E Class AMG as a daily driver. The S Class was just a bit too big for me personally, otherwise it's a great choice. If you do decide on the S Class be sure to get the ABC option. The ABC suspension completely eliminates any body roll and makes the car handle flat and feel pretty nimble for a heavy sedan.
As far as the Cayenne V6, it's a very good SUV but I just wouldn't want it as my all around daily. The V6 is also a bit coarse at higher rpms. You could think about adding one to your garage later; a 911 variant, the S Class, and a Cayenne would make for a well rounded trio.