The Peaceful Lullabies version is more expensive due to being filled with cotton and wool and having a bamboo cover. The other is less natural materials but still non-toxic, free of PVC/phthalates.
They both have the removable cover for washing. But it seems like many people add waterproof mattress protectors (not necessarily with these mattresses, just what I've noticed in general).... is that still necessary with mattresses that already have a cover? Did you buy a separate protector to put around yours? Because I'm thinking in that case, if I did that, what does it matter what cover is beneath the cover I put on top (i.e. why wouldn't I just buy the cheaper one since I'm covering it up anyway?).