I used the ME50B for 3 years, and have owned the M13 for close to a year, and I won't even think about a ME50B.
I like the effects better, you can change the order, you can use the same effect type multiple times, there is an effects loop to jam external pedals in the middle of the chain, there is a looper, a tuner with better visibility and it can be controlled via midi.
The only downsides to an M13 IMO is the lack of a built in expression pedal and no sound on sound function. I hated the sound on sound function on my ME50B, though.
ME50B or M9/13? 
