I've never used either bags, but if a store is nearby that has a Big Agnes I would try them out first. You're going to need a insulated pad of some sort underneath you. Utilizing the Big Agnes' sleeve feature with a pad might be worth something. I tend to slide off my pad sometimes. With that said, if it only going to be in the 40s you should keep in mind of the zipper of the one you buy, because sleeping bag with a full zipper you can utilize it as a quilt. It's much easier to sleep on your side like this than inside of a bag regardless of brands.