In my mind wether 15/20 is enough…is very dependent on the amount of noise your band produces. I Ussually play with civilized drummers, no rock/chugga…..9/10 I’m good with 15w..also carrying the room.That said, our band gigs only in theaters and good venues, and we're not a loud band (a Mike Oldfield tribute band). We only need our gear to be loud enough for monitoring purposes (without losing some cranked up tone, but nothing crazy). Maybe this gear wouldn't be enough in more loud genres or environments.
Ussually I take 2 small amps for a stereo rig..that maybe could also be something to consider…besides stereo…the advantage is you carry 2 manageable units, and you only carry them when you think you need it.
The only reason I own a 60w amp…is for outside gigs with big PAs blasting big squares btw…which I hardly ever do.
Here’s my line up…stJames not in the picture…gives me options.
Lightest would be the stJames, lightest stereo rig stJames + champ.
Fav probably the 2 supersonics…loudest the supersonic 60 + the st James.