I've used Sencha Touch before to provide a nice fat-finger-friendly version of Sencha's Ext-JS user interfaces on the iPad and it's a nice framework for providing rich UI components and also helps you segregate your UI itself into models, views and controllers.
I've used PhoneGap to wrap web-based projects for native device access to APIs you can't get a hold of using plain javascript or that wouldn't be exposed via Sencha Touch - PhoneGap provides an app shell, a UIWebView to execute your web app inside and a lot of nicely wrapped device level APIs accessible to your javascript code.
If you just want the web app stuff, Sencha Touch is really nice but a little heavy compared to options like jQuery Mobile so I'd use it in future for things that are very custom-component heavy like grids and complex forms, and for bigger applications where compartmentalisation is more important - whereas I'd stick to something like jQuery Mobile for just mobile-friendly UI elements like buttons and basic form controls or for a more simple app.
Sencha Touch has a bit of a learning curve compared to typical web app work, as the entire interface is typically declared within Sencha and UI elements instantiated and passed around, whereas with jQuery Mobile or the likes you can get away with just making it up mostly in HTML and throwing in JS components as and when you need them. To me, Sencha Touch is a slightly bigger all-or-nothing approach.
PhoneGap I would use in general any time I had to get access to device level APIs that ST v2.0 hasn't provided or anything else in bespoke native code that can't be done within a web app.
They're different products for different purposes. Using Sencha Touch and PhoneGap together works really well. You can develop your app in a browser using the Sencha Touch framework then simply drop your code into a PhoneGap Xcode or Eclipse project and your pretty much ready to submit to the relevant app stores.
My app doesn't need native capabilities as of now. I'd stick to their native packaging tools - zero learning.
See towards the end - http://vimeo.com/37974749 . The Ext.device.[Camera,Device,Connection,Geolocation,Notification,Orientation] APIs support both Phonegap and their own native packager.
