Digitech Jamman versus Boss RC-20 (non-XL) LoopStation, my opinions:
Same size. Same size wallwart AC adapter.
99 loops vs. 11. Of the 11 loop storage spaces on the Boss, one is marked "single" - that is, the sample does not repeat: only plays once when you press Play. On the Jamman, any arbitrary sample can be marked as "single play / does not repeat". This can be good if you want to carry a ton of sound effects.
Compact Flash card storage, included good for 24 min. (Jamman) vs. 5.5 minutes built-in and nonremoveable/nonexpandable (RC-20),
Stored loops on both can be retained when the unit is unplugged. Buy a large enough CF card for the Jamman and the storage goes to several hours.
Auto-record on Jamman that can be initiated by the extra footswitch. Autorecord on Boss involves bending over and pressing chiclet-sized buttons, possibly in bad lighting conditions.
The old Digitech FS-300 footswitch (current model is FS-3X, same features) that I got to use with my Electrix Repeater (Mk I) works perfectly with the Jamman. With the Boss, the FS-300 only increments *upward* on the loops (so you had to walk over to the RC-20, bend down and turn the knob to go down in the sample/loop list).
The Jamman, the FS-300 does enable autorecord, decrement, increment loops. Jamman has USB jack for import/export of loops to the card without having to remove the card from the Jamman. Supports both Windows XP and Mac OS X. *That's* spiffy.
I plan to keep the Boss RC-20 Loopstation, even though I like the Jamman and if I need to drop one or the other from my rig any given evening, the Jamman probably stays. Reasons:
* I never get rid of gear if I can help it
* the RC-20 can run on batteries and the Jamman cannot
* the Jamman lacks reverse playback mode