I have listened to both and have also listened to several other models of the Shure SRH series. I prefer the voicing on the 840's over the M50's. The M50's bass seems more innacurrate/overbearing and their midrange is lacking when I can compare them to headphones such as the AKG 271. The headfi people LOVE them though. I am in the camp that they are a bit overrated. I prefer the bass on the Shures (and they have tested through 3rd parties they maintain their composure in the bass at high output levels). Try to listen to them both if you can.
They are both similar in comfort. Neither of them are great in that regard and have similar design and features (like how they fold up).
BTW which ever one you get, get the straight cable if you can over the coiled.
