I've seen some DCS raw images as I quite recently fixed some format support issues in RawTherapee. Although they are only 12 bit the files are very clean. Haven't done any side by side comparison but I'm guessing that the real dynamic range difference is very small compared to older "16 bit" backs, as the last of those bits are only noise. So I don't think you'll be disappointed by the image quality.
Just to show an example I've attached a 100% crop of a dark shadow pushed 4 stops from a Kodak Proback645, so you can see how clean the noise profile is (there's no noise reduction applied, not hidden automatic noise reduction either as I've used RT). The black corner is from the same area without pushing. So you see you need pretty extreme processing before noise would become an issue. Noise is well-behaved too, no patterns or something like that.
If you intend to buy private I'd look on this forum and getdpi first, and ebay second. Dealers can have really nice deals too and that can feel safer, but I think buying off a forum and apply some common sense is pretty safe too.
The DCS Pro is 37x37mm 16 megapixels with 9 um pixels. The pixel-peep sharpness will be very high indeed, so you won't be disappointed by that. The M9 doesn't have any super dynamic range so I imagine that shadow detail won't differ that much, but it's hard to say for sure. The look of lenses will of course be different, and if you're a Leica fan you'll of course say that Leica is always better, but it will much be a matter of taste.
Concerning color the Kodak is known for a rich slide film-like look, which probably is quite different from what you get from the M9, what's better is a matter of taste.