If App will contain a lot of HTML with some widgets here and there, choose Backbone.
If App will use mostly Webix widgets, Webix Jet will be better.
Webix Jet simplifies usage of Webix widgets a lot, but it doesn’t care much about plain HTML elements. It was designed as an solution for Webix based SPA. If you expect that big part of the App will use plain HTML, it has sense to use more HTML oriented framework, such as Backbone.
Thanks alot, Maksim!