[Vincent01]

For me, its hard to decide between. They have aspects that make them unique, and good sounding. 

 

[shane55]

SRH840 were downright painful for me. They weighed down on the top center of my balding head and it just would not do!
I don't think they are as portable as the M50 because they are larger and heavier.
I do agree that they are the more 'neutral' of the two, though they have their own issues. I thought they were a bit light on the deep bass. 
 
I find the M50's very comfortable. I just had them on for a stretch, no sweat, no discomfort.
But alas, the M50's are not great in the upper mids, where there is a recession going on. So you'll find some tonality lacking. It's my only gripe about their sound... and it's very minor.
But I don't care. I love these babies. Some have complained about 'metalic' highs. I've never heard that. Might be the source files too... don't know.
As for break-in... mine have improved as the bloat in the bass has gone away completely. 
 
Good luck.
 
shane