Back-End Design: Firebase vs mLab

Hey all, I’m working on a personal project and I’m trying to wrap my head around the back-end design. For detailed background on the app, check out this blog post: The short version of the problem reads something like this:

-Users will be able to sign up for and log into the app. Once logged in, they can re-create their in-game character lineup from an in-app selection of characters. When they hit a button to save changes to the line-up, that will populate several arrays (one for each character rarity level), that the front-end of the app will use to display their roster strengths.

The thing that’s tripping me is up is I haven’t come across a similar back-end pattern in codealongs or projects before, everything is pulling from some API somewhere, or is pure CRUD. I’m thinking I want to do this in mongoose/mLab, but would firebase be a better use case for an app like this? Also, If anybody could point me to articles or books that would help me programmatically model the problem in my head, I would appreciate it.

EDIT: Link to the repo for this project: