I actually think it's the other way around and that the Boss pedals came after the JamMan. At the time that I bought it there were no Boss loopers.
The JamMan is/was roughly the same price as the RC-20XL. It stores a lot more because it uses some pretty standard storage - CompactFlash or in the case of the new ones SD cards. This is kind of handy because I can pull the CF card and just connect that up to my computer rather than dragging the pedal and power supply over to the computer. That plus I can have multiple cards with tracks on it and I don't need to delete stuff so much. The CF cards work ok for me because I've got a bunch of them (my old camera used those and so I have them left over - the new camera uses SD cards). Over the next few years I imagine these will get scarcer.
The new JamMan is stereo and I think that would only matter if you are running a stereo rig and have effects that have stereo out. I have heard complaints about stereo looping before on the JamMan mailing list and it seems like some of the serious looper users are into ambient/industrial/space music and so using a lot of time and spatial effects which are stereo. I can see the reasons for making the looper support that. You might also be able to keep voice (the new and old JamMan have XLR mike input) and instrument separate and so run them through different amps on output.
The biggest new features, in my mind, are the extra switches and the new software JamManager. The extra switches look to be track up and down which are really useful. I bought the extra external footswitch to add those controls but building them in is good. I use it to have different backing loops for different parts of a song and then switch between them. The JamManager software looks to be for managing your loops on your computer for downloading to the JamMan and possibly for various types of conversion of files etc. I'm hopeful that this software will work with the JamMan - it should if the file formats are the same or similar enough.
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