One advantage to building PhoneGap applications is that you can test your application on a physical device before registering with a developer program. Just host all of your source files on a web server and and point the device’s web browser at the server.
$100 per year to enroll in a developer program isn’t the end of the world, but I’d rather keep it until I know I have something to release!
Of course, with this testing method your application will not have access to device functionality exposed through the PhoneGap API, but it is still a good way to rapidly prototype your application’s user interface.
You should test how (the lack of) JIT compilation affects your application’s performance. The most obvious way to do this is to deploy your application to a physical iOS device. There is another way that will let you delay enrolling in the iOS developer program a little longer. Install Google Chrome for iOS. Google Chrome for iOS uses the same UIWebView component that your PhoneGap application will use, and therefore is similarly stunted when it comes to JIT compilation.