I could write on this subject for pages but you'd die of boredom! In brief: Aside from the obvious visuals, the Z4's later suspension, especially at the rear, makes a big difference on the track and skid pad. Also the brakes (E46 M3 Competition parts along with the CSL steering rack). On the road, you really don’t want to approach the limit in either car, especially in the rain! The later Z3M's had the same S54 engines as the Z4M's but with a 5 speed vs 6speed. Traction control is worthless in both cars. Both cars have you sitting far back, almost at the rear axle which is lots of fun with the short wheelbase but way too twitchy to drift, at least for me given the big, grippy rear tires. Both are great, impractical fun! Which one? I guess the sheer weirdness and more analog experience of the Z3M Coupe would be my choice if I were to get one again. But the looks of the Z4M are so wonderful…. No bad choices and on any given day, who could choose? If you don’t have a big grin on your face after driving either one you are a lost cause!
Kent