Having been searching for decent arguments as the merits of both, and my last car in the UK was a 911, here in Denver I've been bewitched by a 2007 Z4Mc. I'm not sure I'd ever not regret going for the M, as the roads in Colorado are pretty fine. So, will likely get my next door neighbour to do his side gig, and head to Vegas with him for an auction and drive back an M. We've just started the search. Otherwise, there is a fine example in Maryland with just 9k on the clock, pristine, from a respected German cars prestige seller.
https://www.avaziauto.com/2007_BMW_Z4%20M_Gaithersburg_MD_22858635.veh
That being said, for $35k less, is an equally well-looked after si: https://www.carsforsale.com/vehicle/details/45343509 Oh, the American way of putting exclamation marks after each feature. Ho hum.
The point being, found this thread to be very informative. And the pros/cons, SWAT analysis, lead to the M. Cheers.