Honestly, you should just stick with XCode. Because it is the native iOS way of developing, it will be easier to get around using this (though some people might disagree).
Sencha touch is used for developing app using HTML5 alone, whereas XCode is the actual complete IDE for all iOS ans OSX applications. You will get a lot with the XCode package, which will make developing easier.
XCode also comes with a plethora of tools for you to use, which will help you in more ways than you could think. From the simulator, allowing you to run you app on virtual iOS devices, to the Instruments which will help you find memory leaks, check your app's energy diagnostics and even an activity monitor so you can watch your devices performance as your app runs.
PhoneGap applications are developed using HTML, CSS, and JavaScript, while letting you distribute your app to multiple platforms, not just iOS. Here is the visual explanation
From the PhoneGap site:
"PhoneGap is a distribution of Apache Cordova. You can think of Apache
Cordova as the engine that powers PhoneGap, similar to how WebKit is
the engine that powers Chrome or Safari. (Browser geeks, please allow
me the affordance of this analogy and I’ll buy you a beer later.)"
From the Apple Developer site:
"The Xcode developer toolset provides everything you need to create
great apps for Mac, iPhone, iPad, and iPod touch. It’s tightly
integrated with the Cocoa and Cocoa Touch frameworks, creating a
productive and intuitive development environment that is powerful
enough to be used by Apple to produce OS X and iOS."
Here is the link for all of the info on XCode.
My advice, if you are making for Apple, stick with Apple.
Welcome aboard.