Note that these are not antonyms. You can get super bright diffuse LEDs, or low brightness non diffuse ones.
I haven’t done much vactrol making yet but I plan to soon. I have some green super bright, non diffuse, flat top LEDs I will be trying, in accordance with some of the recommendations in the discussion starting here.
I certainly would expect this would depend on the LED current determined by the series resistor. I’ve used super bright LEDs as indicators where they would be blindingly bright at full current, but with large enough resistors they are comfortable to look at and draw far less current.