What is difference between IIF and Switch Function

The Iif function returns one of two values depending on whether the expression is true or not. The following expression uses the Iif function to return a Boolean value of True if the value of LineTotal exceeds 100. Otherwise it returns False: 

=IIF(Fields!LineTotal.Value > 100, True, False)

Use multiple IIF functions (also known as "nested IIFs") to return one of three values depending on the value of PctComplete. The following expression can be placed in the fill color of a text box to change the background color depending on the value in the text box. 

=IIF(Fields!PctComplete.Value >= 10, "Green", IIF(Fields!PctComplete.Value >= 1, "Blue", "Red"))

Values greater than or equal to 10 display with a green background, between 1 and 9 display with a blue background, and less than 1 display with a red background.

A different way to get the same functionality uses the Switch function. The Switch function is useful when you have three or more conditions to test. The Switch function returns the value associated with the first expression in a series that evaluates to true:
 

=Switch(Fields!PctComplete.Value >= 10, "Green", Fields!PctComplete.Value >= 1, "Blue", Fields!PctComplete.Value = 1, "Yellow", Fields!PctComplete.Value <= 0, "Red",)

Values greater than or equal to 10 display with a green background, between 1 and 9 display with a blue background, equal to 1 display with a yellow background, and 0 or less display with a red background.