Coming from someone with a 545i GO WITH THE M5... I only went with the 545i because I don't want a super expensive car that I have to give a chrap about. the 545i or 550i is in a different league than the //M which is one of the best cars on the road. If I were you I would go for the //M, it will last you a very long time and you will love it. with the 550i you will probably just "like" it...

 

NO doubt E60 M5 is the best among both.

 

Go 550i. I had the same issue and just don't think the M5 warranted the hype, less power then I imagined coming from an AMG, too thirsty and I hate theSMG. I think a 2008 550i sport looks better too.