The quality you can get from a medium format digital back, even a relatively elderly one, is simply stunning. I've got a Sinarback 54M that I picked up for around $850 on eBay. (It's actually my second one, but I won't bore you with the details of all that.) I originally shot it on the back of a Mamiya RZ67 Pro II, and now I use it on a Hasselblad 553ELX.
It's so old--roughly 12 years now--that you have to shoot it tethered: it has no screen, no battery, no memory card slot or other storage. And it's only 22 megapixels. But it's 22 *glorious* megapixels. It actually kills my Nikon D810 with its 36MP. That's partially because it basically has the same resolution in the end--I prefer a little less oblong crop than the 3:2 ratio of the DSLR, so I frequently throw away bits, especially at the bottom, from the D810's files to crop a bit more square. The 4:3 ratio of the MF sensor is a little bit nicer, in my opinion, and leaves me about the same file size as I would have after cropping a D810 shot.
The older sensor won't necessarily give you the best color and apparent sharpness straight out of camera, compared to a modern DSLR--which has many adjustments sort of baked in, even if they can be tweaked considerably. However, it just feels as if you can do more to manipulate the files to get what you want; it's kind of like, I think, using really good movie film and shooting it a little flat, and then having lots of latitude in color grading it afterward.
To give you an example of what a file from an old back can produce, have a look:
http://www.presquevu.com/apa071.jpg. Make sure to blow it up to full size, and you'll see what I'm talking about.
(By the way, this points out something else--I really think the Mamiya RZ lenses are sharper than my Zeiss Hasselblad lenses. That particular portrait was shot with the 180 W-N, and it just pops a little more than the lenses on my Blad, although they're no slouches either, certainly.)
I wouldn't give up an M9--I'd *love* to have one to complement my M3 and M6--but depending on what you shoot primarily, you might find digital medium format more useful. But if you can afford to keep both, there's no reason not to--and double your pleasure!