To give an example of the point that Chris made about cost difference, Aircraft Spruce's price on 2024-T3 is almost two times the price of 6061-T6.
I would like to emphasize that I use the materials that are called out in the plans. But, If there were a place where the I may think a better material is suitable based on availability, cost or characteristics, I would never substitute without first giving the designer a call to seek his approval.