I am SO lucky that I can keep both cars.
If I could keep only one, though, it would be.........
The M5. 
Really. The Porsche is more fun to drive hard, and a lot less pleasant to drive the rest of the time. I like my M5's room, my quiet, my stereo, my seat heaters, my folding rear seats, my trunk, my gorgeous interior - oh yeah - its pretty fast, too!
If I tracked my car 6-12x a year instead of 1
If I didn't have children
If I didn't have to carry large objects
If I didn't need enough room for my wife AND my suitcases
If I didn't mind the limited range (small gas tank)
If I didn't want to listen to music sans road noise without resorting to way-too-loud stereo
THEN I would probably be very satisfied with the Porsche as an only car.
Still - it is a car everyone who can afford it should experience at some point in their lives!