PhoneGap is a middle-platform which is able to access some native APIs on many platforms and its interface language is Javascript. This means that PhoneGap is able to work with many cross-platform mobile web frameworks, such as: Sencha Touch, Titanium, jQuery Mobile, etc.
Sencha Touch 2 also provides a few native APIs (currently available on iOS, Android) through its SDK.
If you are not going to use any native APIs, then they don't have any major differences on performance and functionality as the only work done is just to wrap your Javascript code into a webview native component on corresponding device.
Lastly, the 2 main differences are:
- PhoneGap provides much more APIs than Sencha Touch SDK.
- PhoneGap supports 7 platforms, while Sencha Touch SDK only 2.