Design difference between target area preview?

Is the difference between showing the area a unit will target as a circularized area or with the jagged edges to more clearly show which tiles will be included purely an aesthetic choice?

Additionally, when determining if a unit will target an enemy on a tile of the board, is this done on a tile by tile basis, or is a hitbox system used?

Do you mean tower range? That’s a pure circle (square for healers).