Too many electronics and driving aids make the newer cars very boring to drive, even if you compare the E39 M5 to the E60, the old M5 was a lot more fun ...
seriously, I wouldn't know which to choose. I have a secret love for the S4 and it's 5cyl turbo, the M5 is just classic M-power everywhere, and the 500E with the nice V8 is just such a monster!