Less than 20W? Give me a 100W any day.
20-40W? I'm not sure. There are too many variables. Music played, overall band volume, headroom needed if any, tone/feel preferences, weight/logistics.
I'm currently playing the 20W jubilee and a 2x12 and so far seems to be perfect for the band I'm in but haven't gigged with them (and this amp) yet, therefore I don't really know if will be good enough. The 20W seems to have a wider sweet spot, doesn't need to run with master very high to sound the best.
I used to play a jubilee with a 4x12 and needed an attenuator to be manageable in the sweet spot (master volume > 6) in a very loud band.