I'm a relatively new owner of an M240 and still as yet haven't sold my M9. Here are my thoughts:
-The M240's jpegs are beautiful. OTOH it is virtually mandatory to shoot DNG with the M9.
-The M240's shutter makes a soft click almost like a film M, and there is no jarring motor sound after the shot as there is with the M9 (even if you delay it with discrete mode).
-The M240's tripod thread is part of the body (there's a hole in the baseplate, a la Leicaflex SL). The M9's tripod thread is part of the baseplate and puts a lot of torque on it.
-The M240 is several ounces heavier than the M9, and although it really isn't much deeper, it feels like it is because of the thumb rest which changes the ergonomics more than you'd think such a little blip could do.
-The M240 lacks a frame line preview lever. Not a biggie if you never used one, but for a veteran Leica M user like myself who got used to using it, the absence is annoying until you get used to it.
-Battery life is better with the M240, however the battery is twice the size of the M9's and costs almost $200!
-There are quite a few reports of the M9's sensor glass either cracking or the coating becoming crazed, or lines of dead pixels necessitatating sensor replacement. Leica has stepped up and replaced them free out of warranty, but their turnaround time can be quite long.
-I can't detect any improvement in clarity or accuracy of the M240's rangefinder as some have claimed. But maybe it's more resistant to going out of adjustment. However my M9 has never gone out.
-The M240 is available in silver chrome finish. To get that in an M9 you'll have to spend on a "P".
-The M240's LCD is Gorilla Glass like some smartphones, whereas the M9's is plastic. If you want sapphire (much more resistant than Gorilla), again, you'll need to spend on an M9P or have an M9 upgraded (if they still do that). Personally I do not and never did think the M9P was worth the added cost when it is the same IQ and a screen protector costs a few bucks.
-A used M9 or M9P will not have a warranty, and if something major goes wrong that Leica won't goodwill, then you could end up paying close to the cost of a demo M240 and not ever recoup it if you sell.
-Supposedly the M240 is better sealed. Maybe it is under the buttons and switches, but otherwise I can't see any gasketing anywhere. I would not consider it "weather sealed" in the way of top-end Nikon and Canons.
-It does have live view, provision for an EVF, and a video mode. Personally I won't ever use the latter, and the latest firmware allows disabling of it's button as well as the button for live view. I really don't care much about live view, but it does let me mount my 400mm Visoflex lens, and use M lenses with extension for close work. It also lets me use my old Nikkor lenses. I still prefer to use manual lenses on a traditional SLR, but it's a convenience. The Leica EVF is hell expensive, but the Olympus EVF2 works and is a fraction of the cost.
