A big difference in the output between the new M and something like a Nikon will likely be the color filter that Leica uses on top of the sensor, assuming they use something similar to the M9's. Leica (and, btw, Sony cameras) tends to focus on good color over great low light performance, whereas Nikon and Canon tend to use weaker color filters to improve low light performance.
Another thing to consider is, while the M will surely perform better in lowlight than the M9, it'll likely also have better dynamic range at low ISO. We'll see. Either way, digital cameras are to the point where they're all pretty good to me, so upgrading would be like trying a new film, which, although it sounds fun, it doesn't sound all that enticing for another $7K. Unless the new M's output is outrageously preferable to me, I'll likely stick with my M9 until it dies. I rarely print over 13x19, anyways, so the M9 does a great job.
