I have owned a Z3M and currently own a Z4M
Both are AMAZING cars with more similarities than differences, but there are differences.
Believe it or not, both feel just as raw. The Z4M is a little softer and more comfortable though. The Z3M gives you more of a small car (Miata) feel. Both feel amazingly connected to the road. For a big guy like me, the Z4M is better because of the extra room. My head used to stick up over the Z3M windshield and I would have to pick bugs out of my hair.
Speed wise, the Z4M is faster. But not so fast that the Z3M is slow in comparison.
You can't go wrong with either car and both are destined to be absolute BMW classics!!!