There is actually a comparison of the two in the official WebStorm FAQ. However, the version history of that page shows it was last updated December 13, so I'm not sure if it's maintained.
This is an extract from the FAQs for reference:
What is WebStorm & PhpStorm?
WebStorm & PhpStorm are IDEs (Integrated Development Environment) built on top of JetBrains IntelliJ platform and narrowed for web development.
Which IDE do I need?
PhpStorm is designed to cover all needs of PHP developer including full JavaScript, CSS and HTML support. WebStorm is for hardcore JavaScript developers. It includes features PHP developer normally doesn’t need like Node.JS or JSUnit. However corresponding plugins can be installed into PhpStorm for free.
How often new vesions (sic) are going to be released?
Preliminarily, WebStorm and PhpStorm major updates will be available twice in a year. Minor (bugfix) updates are issued periodically as required.
snip
IntelliJ IDEA vs WebStorm features
IntelliJ IDEA remains JetBrains' flagship product and IntelliJ IDEA provides full JavaScript support along with all other features of WebStorm via bundled or downloadable plugins. The only thing missing is the simplified project setup.