Unity - when nothing distracts from the whole you have unity. Unity without variation can be uninteresting - like driving through western Kansas on the interstate. Unity with diversity generally has more to offer in both art and in life. Opposition - based on contrasting visual concepts. Sorting and identification activities help them learn to focus on learning tasks.
Balance is the consideration of visual weight and importance.
It is a way to compare the right and left side of a composition. Both sides are similar in visual weight but not mirrored. It is more casual, dynamic, and relaxed feeling so it is often called informal balance. Radial balance is not very common in artist's compositions, but it is like a daisy or sunflower with everything arranged around a center. Rose windows of cathedrals use this design system.