Is it fair to compare?
I have now driven the '06 //M and an '11-3.5Si. I think if money was not an issue I would definitely prefer the 3.5Si. The car seems to be developed to the point where it's performance exceeds what I could accomplish on a public road... Just like the //M.
I didn't look, does the '06 //M have the S/SS mode selector?
I still have to figure out how I am going to be able to drive any Z throughout a winter with snow and LOTS of rain.