I have a 550i (not quite M5 but close enough) as a DD and the V8V as the weekend car. As others have said, comparing the two are like comparing apples and oranges. Personally, I enjoy both of the cars but for different reasons. The 550i is luxurious and still has ton of power. The AM is doesn't have the power IMO but it definitely has the "beauty".

I use the 550i to take out clients and I use the AM to take out my wife. It really comes down to weather you want to feel "special" b/c you definitely will get the thumbs up when you drive the AM.

If I HAD to choose one, I'd probably go with the M5. Just my opinion so don't kill me! I still love AM, as evidenced by my owning one