A good question to ask about the cotton batting is how well it holds up over time, and whether it settles over time or becomes compressed and loses some of its R-value. Also check to see how rodent and bug resistant it is.

Now, you may find that it doesn't hold up as well as fiberglass and decide to go for it anyway, with the knowledge that it's better for the planet and in however many years' time you'll just need to replace it. But it's good to know, in any case.

If those things are issues, you might go for fiberglass in permanently enclosed spaces and cotton in any spaces you're likely ever access or that are open-ish, thus combing durability and safety.

And again, the caveat: I so don't know what I'm talking about.

Dan