The answer to your question is actually easy for me. I would keep the M5. Both cars have so much performance its hard to use it on the street. But the M5 does so many more things well, and it doesn't stick out. I find myself worrying about tickets, door dings, and having to avoid curious drivers more in the Porsche.
But as long as I can keep both, I get to enjoy driving the Porsche when I can.
Real cool comparison... thanks for making the rest of us (who don't own two of the world's finest automobiles!!) very jealous. In all seriousness, great write up. I am very happy for you. I hope to join you in this prestigious club some day soon?!