I was working on making a mobile web app a few weeks back and after quickly deciding that a js framework would be better than a responsive site (lots of widgets, built-in ajax functionality and push-state) I tried both jQ mobile and jQ Touch[0]. Both are quite similar in most respects(learning curve, widgets, etc.) However, if you're looking to completely overhaul the look then jq Mobile, IMHO, is not that great. I found the themeroller[1] to be clunky and difficult to use and would've preferred writing css. jqTouch, on the other hand, is quite customizable (allows SASS based theming).
[0] - http://www.jqtouch.com/