Dual rate lessens your total throw so if you set it to 75 left and 75 right you will only get 75 perecent of the mechanical limit of the servo.
This will have the effect of making your controls less sensitive.
Expo will make them less (or more) sensitive around center but retain full throw at the ends of stick movement.
Futaba uses negative expo to soften the center.
So with no expo, moving your stick 50 percent to the left results in 50 percent of left hand servo movement. Dial in some expo and the 50 percent of stick movement might result in only 30 precent throw while the 2nd half of the stick movement moves the servo the remaining 70 percent of total throw. The advantage is hovering is smoother while retaining full throw for stick banging aerobatics.
Try 25 percent or so and see.