Load of rubbish I know which one I'd be in for speed if my life depended on it.
If you have a look at the Top Gear lap times the Z4M is quicker than almost everything in a comparable price bracket only the new M3 and Evo FQ 400, Evora and a couple of TVRs have posted better times. Everthing faster is either a track biased car or an exotic from the GT-R upwards. The old M3 was seconds behind but of course it might have been wet as it was when they drove the CSL.
Lap times posted by magazines and TV programmes are only good for one thing and thats telling your mates down the pub that your car is 4/10ths of a second quicker than an Enzo round a given track. The reality is in a traffic light GP there wouldnt be much between the old M3 all the way to the limiter and maybe even the the new one upto about 120 MPH. Unless you are in an actual race where a tenth here and there would make a difference then I wouldnt lose any sleep over it.
At the end of the day park a Z4M Coupe in a car park next to an M3 and sit smuggly in the beer garden as the Z4M gets all the attention and the M3 gets ignored for the mass produced car (albeit a very good one) that it is.
I had a conversation a few weeks ago with the owner of a 996 GT3 who was explaining how fast he was round Outlon Park. When I told him I was only 1.5 secs a lap slower in my old XR2 his jaw hit the floor.