Obviously, we’re going to be biased, but here’s a quick comparison between Composer and a few other projects.
|Dependencies||History.js||jQuery or Zepto, underscore.js|
|Ships with a generic class system||Yes||No|
|Provides hierarchical data relationships||Yes||As a module|
|Provides filtered collections||Yes||No|
|Provides controller <-> collection tracking||Yes||No|
|Provides AJAX API syncing out of the box||DIY||Yes|
|Allows granular silencing of specific events||Yes||No|
|Supports using promises for async operations||Yes, via ES6 (or compatible, like Bluebird)||Yes (via jQuery promises)|
|"View" or "Controller"?||Controller||View|
|Router supports named parameters||No (regex only)||Yes|
|Router provides automatic link binding||Yes||No|
|Comes with history management||Requires History.js||Requires jQuery|
|Supports event binding contexts||Yes, string-based||Yes, object-based|
|Provides Controller event inheritance||Yes||No|
|Controller DOM diffing/patching/batching||Yes with morphdom||As a module w/ Marionette/VDOM|
|Supports IE 6+||When using Mootools||No|
This list is not exhaustive. For instance, the relational models that Composer and Backbone use are fairly different in the way they handle data (Backbone forces one-instance-many-locations, Composer favors many-instances). There are also utility functions in models/collections that one framework provides that the other doesn’t and vice versa.
The best way to get a good feeling for Composer’s abilities is to read the docs.