From what it seems, I believe the objectives of both the projects differ greatly. js-git attempts to replicate the functionality of git entirely using Javascript, whereas this one is to make people learn and understand git using programming paradigms.
One of the major factors in the debate was community support. Marionette is built on top of Backbone, which meant depending on support of two separate projects. Ember.js, on the other hand is by a single developer. Kind of like why people prefer buying Nexus phones. They are the first ones to officially get updated with the latest Android version. Others might take months before the handset manufacturer decides to update them.
Also, we were certain that we needed view hierarchies, and Marionette provided them too. Back in the day though, it was still in its early stages, and we couldn't really count on it. "What if an API change broke our app with the next upgrade?" we thought. Ember.js on the other hand was pretty much frozen and as in a 1.0 pre2 version. Also, there were many options available, Backbone+Marionette or Backbone+Thorax? It was confusing.
Part 1: 10 day retrospective https://blog.sheepingaround.com/post/182394390565/sheeping-a...
Part 2 (currently the url): 1 month retrospective https://blog.sheepingaround.com/post/183090206855/one-month-...