as far as i remember, the M12 has a much faster/ better midrange. Vocals can be stunning.
NHT classic 3 - I have only heard them twice, but they seem to be 'darker' sounding, heavier bass (perhaps less accurate than the M12). Highs may seem rolled off by comparison to the M12, but i've not listened to them side by side, so the jury is still out on that one.
M12 has excellent PRaT. NHT less so.
The two have much different 'flavours', no question