40 Watt is the one they refer to when they say high temp on the videos at FT.
A 20 Watt will work OK for short glue runs.
I tried a 40W the other day but it did not seem to like the multi temp glue stick (flow issues if I left it on too long, and, the glue was boiling full of bubbles when it came out).
I plan to go back to my 20 watter and get a second 20 watt and have it hot and ready if I do something with a long glue run. It just seems easier to me to switch back and forth than deal with the 40watt boil.