Different Speeds

I was throwing with a radar ball type thing. It says its pretty accurate and its a 2.0 ball. I guess it should be close to the actual speed of the ball and these are the numbers it gave me.

Fastball (4seam): 75-77

Change-up(circle change): 62-64

I can also throw a change-up 56-58 constantly by taking a little off of the change-up.
My question is would it be more effective to thorw the change-up 62-64 or 56-58.

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.