Each to their own I suppose, there quite hard to compare really, both for different purposes.
The M3 is bigger and also quite old now... the new one will wipe the floor with everything like the new M5 if im going to be honest I cant wait for the new one. Couldn't think of anything better than running a Z4M and an M3 or M5 together perfect combo in my opinion.
I love the 1M but theres something 'bodged' about it, an incredible bodge but still a bodge and its like a baby... im definitely in the M3 crowd, but if i were to let the Z go it would be for a Porsche, but not a Cayman although fantastic its the same thing would have a 911 over one any day.