Great comparison pics. Hmmm, tough call. I like the rear of the old better with the exception of the duckbill. The "new" rear is too plain for me, too Accord-like (my same complaint for the new 3-series).
From the side equally, although I hate the gull/gill inlet thing on the new Z4 in front of the side BMW badge.
From the front the new one definitely, although I hate the silver wind deflector things on the lower grill. I think the M looks better/more aggressive than both from the front.
Interior hands down to the new Z4.
I am surprised at how well the old compares to the new, and the old angular flame surfacing keeps it looking just as modern in my opinion. They both are striking cars.
I guess I like the New one better, but like the M-Roadster better still. Then again, I own an M-Roadster. The new car looks good, but I see no reason based on looks or performance to upgrade. I'll re-evaluate when the new Z4M comes out in a few years.