MMEM5,
Set the money issue aside and ask if your heart will be happier with newer tech, at the expense of a car that will never handle like an M, or will your heart only be happy with a worthy replacement for the car you just lost. Based on your post, it sounds like you will be most content with the M5. The tech gains and new-model-car gee whiz will only last a little while with the 550, whereas the visceral enjoyment of your M5 will be there every time you start the car. Plus, it is an M; no 5 series will ever be a true M.
Good luck, and when you get your car tomorrow, be sure to post pics
Regards,
Danny