I agree with Sardis. The Z4 and the 370Z are not even in the same arena. They are completely different. In the nissan/infinity worth the only thing close to compare with the Z4 would be the G35/G37 IMO.
Even then, the G wins! I just got rid of my G35 to get a 370Z for that more nitty gritty sports car.Test driving and price comparison would seal the deal for most though. If you are married, the wife might lean more towards the Z4 cause it is "CUTE" HAHAHAH!