Pet food quantity - large package or small package?

We usually get the medium sized bag for our two cats. The bag would normally last a week so we don't have to really worry about it going bad before using it all. Plus, it's included when we do our weekly grocery shopping. Another thing we get is two of the small bags of treats for them, of course different flavors and combined the two bags into a sealable container. The treats will last a long time and they get something different or the same each time so it keeps them guessing the flavors.