jQuery Mobile supports more number of mobile platform as compare to Sencha Touch. Sencha touch only supports iOS, Android and recently Blackberry.
jQuery Mobile and Sencha touch both provides great UI features with lots of control but Sencha touch wins here. Sencha Touch offers a bit more like lots of icons, or built-in maps.
jQuery Mobile is easy to learn but for Sencha touch one need to put extra effort to learn it. In fact the documentation of Sencha touch is not comprehensive but jQuery Mobile documentation is quite good.
jQuery mobile is light weight compare to Sencha touch.
Sencha Touch supports a more MVC style application design, whereas jQuery mobile will simply be a load of markup and a load of jQuery script converting your HTML elements into touch friendly interface components.
jQuery Mobile Framework is easy to integrate with other technologies.
jQuery Mobile is free where Sencha Touch is available free of charge for commercial and open source application development. However, embedding Sencha Touch in a web application builder or software development kit (SDK) requires a paid commercial OEM license agreement. Read more about Sencha Touch Licensing FAQ.