I had a prefacelift 3 litre, loved it, sold it and regretted it so after a couple of other cars, I wanted back into a Z4 and only the M would do as I#d always be wanting the best possible. They're bargains at current values.
Insurance differences were minimal, less than 10%. Buying the car around 18 months ago, the difference between two similar aged cars, M and non M, was 1-2k. It seems in that time, the M has depreciated less than the 3.0si
Tax is a pain, fuel isn't as good as toms figures for the 3.0 as far as I know. For me, comparing to the older 3.0, the M gets roughly 5mpg less, for any given type of driving.
Noise on a cruise is the same, it's only when you put your foot down do you get a different growl from the motor. You do notice the extra power, the M edges more towards animal :evil:
Steering is much better in the M, as is the feel of the LSD. Ride isn't all that different, in fact the 3.0 is worse if it's on run flats.
For me it seems a lot of peoples non M's don't make much sense on this forum, they're often paying to put M bumpers on them, quad exhausts, M sport seats, xenon conversions, m wheels....
