I started with a used M9 in late 2010 when they were still difficult to find new, and lenses were really scarce (at sane prices)... then early this year I finally took the plunge on a demo M240 (and saved about $1000).
Between the two, you're not going to see significant differences in image quality at lower ISOs. The M9 will have a punchier look due to it's narrower dynamic range (it's quick to clip highlights), whereas the M240 has a somewhat flatter look, but can be easily tweaked in post to match the M9. As my processing tastes evolved with the M9, I found I frequently lowered its contrast quite a lot, so I guess the M240 is a good fit for me. M9 files will look sharper out of the box, but my argument is that's somewhat due to fine detail aliasing that gives the impression of higher than actual detail/sharpness through 'false' image detail. The M240's files in comparison, look a bit softer, but take sharpening very well. At higher ISOs, which you indicated wasn't such a priority, the M240 has much better control over colour noise and is pretty competitive with current generation cameras.
Which one to go with, I think will depend a lot on your shooting style. Particularly, how you work with the camera.
I come from 20+ years of SLR/DSLR photography and as a result, shoot a lot and rapidly during outings/events. I always found the M9 a bit laggy and sometimes like I was a half step behind the action while waiting for it. Sure, a lot about rangefinder photography is mastering the art of anticipation, but some things are difficult to predict and I value a camera that is ready quickly for the next shot when 'under fire.' In this respect, the M240 is significantly better and more refined. First of all, it doesn't sound terrible like the M9's clunky-buzzy shutter system. It's much smoother and less noticeable/distracting. And it's way faster at getting to the next shot. Even though on paper it's only a difference of 2 fps vs. ~3fps, it feels like a world of difference to me. Having recently gotten back into shooting weddings, I primarily use the M240 along with a DSLR, with the M9 along on occasion. Whenever I shoot with the M9, it's immediately obvious how much slower it is. And the buffer is much shallower too. Then there are a host of M9 quirks, at least with mine, such as random, progressively worse image banding as the buffer fills and the camera is under greater electronic stress. Not to say the M240 is perfect... It's not. During every wedding it lock-ups 3-4 times where I have to reboot by popping its battery. I also have to force myself to remember not to press any buttons while the buffer is cleaning to the card, as it will lockout the shutter release and sometimes will freeze the camera. In contrast, with my Canons, I can review an image whenever I desire, which is handy for immediately confirming proper exposure in tricky lighting situations, etc..
I have never used an M8, but I suspect the M9 is very much an M8, but full frame. In this respect, if you liked the M8, you may be right at home with the M9. The M240 is a pretty big step up (improvement) in terms of Leica's electronics capability and functionality. But image quality differences will be splitting hairs, for the most part.
If you're a fairly casual shooter, rarely in a rush and often at base ISO, then a used M9 might be the way to go, since they can be found in the low 3000s.
That said, I'd love to pick up a second M240 in place of my M9 for use during gigs....