Note: These scripts are intended as a last resort to send specific code to WebKit-based browsers. This ADC article explains why browser sniffing is discouraged, and describes some alternatives.
Normal vs. XL
Most people should only need the normal library. It detects what version of WebKit is being used and whether or not that version was shipped as a system version of WebKit or a nightly.
If you need to know not only what version of WebKit, but also what WebKit-based browser (Safari, OmniWeb, Shiira, etc), then the XL version is better suited for you.
This matrix of WebKit versions and Safari versions is useful for determining exactly what versions you need to look for.