I have had my Z4M Coupe for almost 6 years now, almost 84K miles, about 87 track days, done the Texas Mile in March 2010 (155.4mph), the GA 1/2 mile this past weekend and it has been amazing journey. I am still not bored and the car challenges me everytime
I have also driven a 350Z Sport (I think) and a G35 6MT. On the latter I helped a friend set it up with suspension, BBK etc and it was a great car and very capable but neither ever felt as sharp as the Z4M.
That being said, drive both and see what you like more. Also, a tracked Z4M may need maintenance like rod bearings, water pump etc so plan for that.
Good luck!