I used to have an 07 Z4 M Coupe. I sold it and bought an E90 M3 since I now have a baby boy to drive around
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I can tell you that the MCoupe was more nimble and definitely had a lower CG compared to the M3, this meant that I felt more comfortable going fast at triple digit speeds.
Although the M Coupe did have a really stiff clutch, poor visibility and mild understeer.
The M3 is a chunky less nimble car that you sit on (not in), but man the V8 is so much better than the old zingy I6. Also, the M3 has so much more versatility with the Tech package (EDC, M Drive, etc..).
Personally, I do miss the M Coupe as a more raw, nimble true sports car.
However, the versatility, motor and capabilities of the new M3 make me forget very quickly...