While both WQL and XQL are proprietary, XQL is slightly less so and close(r) in syntax to XPath. Neither is a good substitute for direct XPath support, in my opinion.
This thread might be helpful. The XQL stuff starts at the 7th post.
Basically, WQL is only available in Developer (queryXML, queryXMLnode). XQL is used in TN also, and trigger filters (IS 6.*). So you might as well just learn XQL. No reason to learn both.