Phonegap Build "is in the cloud" and makes it easy to collect all the goals at once. You just upload your html / css / js to a zip file and they create it on all supported platforms. You can then download the resulting binaries to the appropriate app stores.
The phone screensaver launches on your desktop, and you need to have a means of developing the environment for which you want to compile. So, for iOS, you need Xcode. This allows more control over the configuration and more access to plugins due to greater complexity.