@djmueller Thanks for reaching out!
While I've never used the Marmot Helium Down Sleeping Bag, I have had several climbing partners who have used it over the years and swear by its performance. It's a great sleeping bag and the 15 degree rating makes it very versatile for a range of temperatures. We'll tag some of our users who are experienced hikers on the AT or are looking to do it soon. Hopefully they can lend some insight as well:
Also, a fun post from our community is Changes to the Appalachian Trail. Feel free to take a look.
Hopefully we can get some good discussion going for you. Thanks!