I've seen rm20s go as low as 280-300, whereas rm50 bottom out at 380-400
Apples and Oranges....
rm20
+great for low volume
+very light/portable
+Boost feature not available on any other mts- super cool for solos/studio work
+direct silent output option will allow recording with headphones/IRs
-not quite enough volume for a live band (others willl disagree). The boost will get you there, but that's not a rhythm tone for me
-not as Big & Punchy as an amp with 6l6s or other big tubes
-only 1 module capacity at a time may get tiresome once a collection is acquired
rm50
+Loud enough for ANY situation
+relatively light for a Loud amp
+handles 2 modules
+nice pan reverb
+volume boost feature (unavailable on other mts)
+can handle pretty much any Power tube you need
-volume boost will blow your head off if not set properly
-verb loop needs an on/off feature- there's a guy here who can help you with that, ak from Tenn
In general, the mts modules- especially the modded- sound decent at low volumes. For reference, I had Marshalls & a Mesa that sounded dreadful quiet; once loud they were inspiring though.
However, if you value low volume playing, the rm20/22 is the way to go
Good Luck