Pizza Box first is more optimal.
If you use the Pizza Box and do a mission less than 100% energy, you may get a Pizza which would allow you to use the remaining energy on your next mission, which again, would have a chance at a pizza drop. Even if you do a mission at 100%, if a Pizza drops you are further ahead as you will have gained energy during the 100% mission time and can use that to do a shorter mission "for free"
Having the Pizza Box available makes the luck of getting a Pizza drop more efficient when it happens.
I like having both 😀
!PIZZA