I can't comment about the headphones sonically, because it's been too long since I last heard the SRH-440 to compare. I've also never heard the M50.
Physically though, The SRH-840 is built like a tank. Literally. It can take a lot of abuse and shrug it off. Unfortunately, just like a tank, it is extremely heavy, and it's something you notice after a while, and it does make some people uncomfortable. The headband is also very awkward since it stretches outward and pretty much only the midpoint of the headband contacts your head, so all the pressure gets focused there. The height adjustment thing is also very loose, although not quite to the point where it's useless. There's also somewhat of a clamp, but even with that, the headphones manage to fall off if i move around too much. The 440's fare slightly better in this regard if I remember correctly, since they were lighter.
That said, I preferred the sound of the 840s over the 440's myself. I'm not a basshead by any means, and excessive bass (a la PX100) give me headaches, but the detail it provides competes well with higher end headphones, and the bass is pretty well controlled. It is also definitely faster than my HD-600's while retaining a very similar sound signature, so I find myself alternating between the two depending on my circumstances and music.