if you want to really know, rats are better their bones are denser, so more calcium and the snakes grow faaster. but a warning do not live feed they tend to injure the snakes, even a baby rat.
and 20 gallons is not a good cage for rats, and it will be more expensive than a used rabbit cage. the rabit cage will be larger and able to acomodate more babies as they grow, o at least that is what i have seen in my rodents.
find your local web site for buying and selling used goods, same idea as ebay but n auction and it is local, lots of good deals on there, i see rabbit cages for 10$
my mice are in a 15 gallon, and i have humidity problems, even with a grill screen, so i recomend if you dont like the smell of feces and amonia dont get an aquarium.
and first things first before getting rats, separate your bps, i know here in mtl, there are petshops that sell a 10 gallon terarrium for 10$
and as for the cover you can make one from wood or aluminum corner trims( or gyproc corners bent and fitted together) and fine square mesh chickenwire available at your local hardware store.
if you are serious about keeping bps and want to provide the best care, dont keep them together in too small of a cage,
take 3 rats put them in a 10 gal aquarium and they will fight constantly, put them in a larger cage and they get along well together. the same applies to all animals, including snakes and humans.