Having used both for some time now I am convinced that both can produce stunning IQ.
The main difference - IMO - is the user interface.
AF vs MF
focus peaking vs rangefinder
OVF vs evf and display
I constantly cant decide which one to bring
I like the swivel display of the Nex for discrete shooting
I like the AF for fast shooting
its nice you can do one handed shooting and your are very quick
I sometimes like to have video
but then pro Leica M:
...the ovf of the M9 is so much better in bright light
...I am irritated by too many functions and buttons in the Nex while I can control my M9 during sleep
so what do you thin about the Nex vs Leica M user interface???