IBM Worklight is a full platform for development. Many key features can be accomplished within Worklight that you will not be able to do with a PhoneGap library.
PhoneGap is a software development framework by the Adobe System, which is used to develop mobile applications. To develop apps using PhoneGap, the developer does not require to have knowledge of mobile programming language but only web-development languages like HTML, CSS, and JScript. PhoneGap produces apps for all popular mobile OS platforms such as iOS, Android, BlackBerry, and Windows Mobile OS etc.
PhoneGap library and IBM Worklight both provide HTM5 and CSS support.
IBM Worklight provides a mobile browser simulator and a WYSIWYG Editor.