PhoneGap-BlackBerry-Widget uses the BlackBerry Widget SDK to support BlackBerry OS 5.0 & 6.0. WebWorks is basically Widget SDK 2.0 and it is intended to support OS 6.0 specific features.
The advantage of using PhoneGap-BlackBerry-Widget over WebWorks is that you can port your application to other platforms that are supported by PhoneGap.
You can still port an application that was written using WebWorks/Widget SDK, but you will need to switch out the BlackBerry-specific JavaScript bindings for the PhoneGap JavaScript bindings (e.g. invoking Geolocation, contacts, or accelerometer).