I had a '05 G35 coupe 6MT for 3 years.
If the Z4M still has warranty left, maintenance is free and you get a free annual oil change. Maintenance warranty also includes free brake/rotor replacements.
Over the period of 3 years, my G35 coupe averaged about 0.8mpg better than my Z4M. In my Z4M, on long trips I can get 27~29mpg on the freeway at about 60~65MPH. I get 21.5~22.5mpg in my daily commute (about 75% freeway with a little stop and go). Even when I drive it aggresively it never seems to go under 19mpg. Maybe I don't drive it hard enough.
At 60mph in 6th gear, the G35's at about 2100RPM; the Z4M is at about 2300RPM.
This might sound stupid, but the Z4M has souls. The G35 coupe served me well as a daily driver, but it was
just another car. Boring, heavy, and no souls.