For me there is no question that if I had a choice between an M9 and NEX I would take the M9. It is hard to see that even being a question. Where the NEX is weakest with M lenses is where they really shine on the M cameras -- wide open -- although there is no smearing, the look is often "flatter" than on the M9 files I've seen. Also, the out-of-focus rendering is sometimes less pleasant than for the same lens on an M8 (and I assume M9). And of course, M cameras are simply as fun to use as a camera gets and built to take some abuse. And this is without even getting to the issue of many wides just working better on the M cameras, presumably due to microlenses.
I would say, though, that with certain M lenses at certain focal lengths I have been able to obtain results with the NEX that I am quite happy with; it looks signficantly better than NEX+Kit or m43+M lens. I.e. in the situations where "it works", the file is not put to shame by comparison with the M8, especially when you consider the added flexibility in post.
Of course, there are also many times where I know the M8 would have done better, particularly at wide apertures, and I'm sure the M9 even better. Ideally, I would have an M9 plus an NEX as a very-low-light or backup cam that makes me look even less noticeable, i.e. like a tourist with a point-and-shoot. Unfortunately, I can't justify running an M9 at the same time as the A850 and with landscape my priority I am sticking with the A850 for now. But I would trade several NEXs for an M9.