The difference is very thin between the two cameras, and this is because M9 is already a very good camera up to ISO 1250.
I see 2 advantages of the M here:
1) we can bring M to higher ISO 3200 with an image quality that looks very much like ISO 1250 on M9, plus advantage of 24 Mpix instead of 18 Mpix.
2) M is less prone to clipping highlights easily at high ISO as M9 is doing (see picture below) and it is particularly evident.
Some M ISO3200 vs M9 ISO1250 pictures of different parts of the images I published before:
But anyway life is not so perfect in Leica’s world and there could be a major drawback at ISO 3200 on Leica M: some light banding is appearing on the picture. But firmware is young, and maybe this will be corrected in a future release… (that’s only to reassure myself)
Example here below (I need to mention that I pushed exposure to see it more clearly, but I remarked this problem before, without doing this trick):