Sencha Touch is the most obvious option here, jQuery Mobile is still no where near the quality of the Sencha Touch library.. although jQuery mobile degrades a lot more gracefully on non iOS and Android devices.
Seeing as you are aiming to support only Android and iOS then this shouldn't be a problem (Sencha Touch also supports BlackBerry OS 6 on the Torch 9800 handset too).
I found the responsiveness of jQuery Mobile to be supbar, and there are still a lot of rendering issues and kinks they need to iron out. I wouldn't expect a stable release until early 2012 at best.
You will find Sencha Touch to provide a more "native app" experience too, with a lot of great learning resources that you won't find for jQuery Mobile yet.
One final note, Sencha Touch supports a more MVC style application design (should you choose to go that way with your app), 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.
TL;DR - Sencha Touch is the more complete package providing you want to put in the time.
