I had a 06 Z4 3.0i convertible MT. Right now I have a 2010 370Z 7AT with sports package. Here is a summary of what I think about the 2.
Winner:
Performance:370z
Ride quality: 370z
Comfort: 370z (I'm 6'3")
Visibility: Z4
Noise: Z4 (Do not overlook this, especially if it's going to be your daily driver)
Cargo Room: Z4
People gawking: Z4 (especially from women, my 370 only gets looks from teenage/young adult males
)
Engine Sound: Z4
Exhaust Sound: 370z
Audio quality: Z4 (Keep in mind both I had were stock with no premium packages)
Interior Design/Quality: 370Z
Exterior Design: 370z (however, I'd think its a tie compared to a M series)
Fun to drive: Tie
I'd say pick the 5 categories most important to you from the list and you will find your answer. Hope this helps.