Lord Fluff
Established
The 'better high ISO performance' of the M9 is over rated.
Just to offer a dissenting voice - as someone who shoots in low light frequently (I'm a wedding photographer), I disagree strongly with this.
The M8 at ISO 1250 can produce awful banding, and in any case is borderline unusable. The M9 at ISO 2500 produces (mostly) entirely saleable images. YMMV of course, as will how much this matters to you. For me the M8 was largely useless as a wedding camera, the M9 is so much better.