I currently have a Z4M and previously owned a 2006 350z 300 GT.
Dynamically the Z4M is quite far ahead. The engine is an absolute peach and it is faster and sharper than a 350z.
The best was I can describe my opinions of the engines is that the S54 feels like it was designed for its purpose as a high performance engine whereas the VQ35 was designed for a SUV and modified to be suitable for a performance car.
The only disadvantage with the running costs is you have to factor in the valve clearances being done every 28000 miles apart from that the running costs/mpg are similar.
My 350z insurance was £800, Z4M £780 so the same give or take. Interestingly an E46 M3 quote was £1400.
As with the 350z you need to throw away the standard rubber (RE040-350z / Conti M3-Z4M)
The 350z is more comfortable on long journeys and spacious inside – both boots are equally useless although both can easily be used as daily drivers.
Also do not go in a normal Z4 and think the Z4M will ride and handle the same, they are very different.
The 350z looked better but the Z4M drives & goes better.
A 3.0Si Z4 (260-odd BHP) was equal (give or take a car length) to my revup 350z
