Throwing your full arm speed change is the most effective pitch.
However, if you feel that the hitter is ripe for the super-slow change, you can throw it once in a while just to get them on their front foot. Just remember that if you throw that pitch all the time, good hitters (and as you get older, even not so good hitters) will pick up the difference in motion and tattoo your super-slow mojo.
Making these types of in-game decisions requires that you and your catcher are on the same page. That's why I recommend that you work with the same catcher as much as possible, including warm up throws prior to the game, whether scheduled to pitch that game or not.