Help with Sewer Donut size

So I will need to buy a sewer donut for when we are in Florida next winter. Amazon has two that are add on items made by Camco. They are different sizes and one is twice as much money. Which one would be the right one to purchase or won't it make any difference?

Camco 33912 2.1 x 4.8 x 4.8 inches
or
Camco 33913 2 x 5 x5 inches