HI There
Well - I have both, and I use the M240 much more than the Monochrom. I think the black and white conversions with the M240 work much better than with the M9.
Marc and I have different feelings about the colour - he doesn't like the slight IR sensitivity - but I think the colour is fantastic for nature (better than the M9) and that the skin tones are better than the M9 at high ISO (low ISO is a wash).
But - if you're in doubt, the real difference is the camera - the much quieter shutter, shorter lag, water sealing, better battery life, greatly improved rangefinder and high res LCD on the M240 all make it a pleasure to use, and Live view can be useful if you're shooting wide - it also allows you to use R lenses, Contax and many many other fine lenses that you can't on the MM.
I think I'd say that if you almost never shoot colour, and you're mostly shooting people in lowish light, then the MM is worth considering - for any other circumstances, then the M240 is a bit of a no-brainer.
All the best