The cat's meow comes with a really nice, durable, ripstop nylon compression sack, so there's really no reason to upgrade unless you want waterproofing.

If you want a waterproof one and are willing to spend the money, I'd highly recommend the Sea to Summit eVent Compression Stuff Sack. Its an extra $35, but they're absolutely sick. You'll want the 15L.

Trash bags are a less expensive option.