Just looking for some insight- when you're scouting your next opponents' offenses and defenses, what factors do you use when choosing plays for both??

I scout based on most used, with some exceptions I'd like to keep to myself.

Ditto for me.
One thing I can note is that JDB has said that, even if your overall familiarity with your opponent's plays is up to 100 percent, you will still derive an in-game benefit from having scouted the particular play they are running.