Since it was introduced in November 1999, I have found that XSLT, the XSL Transformations language, is one of the most useful (if not the most useful) tools you can use to manipulate XML documents. Many available APIs and tools work with XML documents from Java or other languages, and I have used many of them in different projects, but cannot recall an XML project that did not use at least some XSLT.
By design, XSLT 2.0 and XQuery 1.0 have a lot in common. Both languages are based on the same foundation: XPath 2.0. Both languages are intended to manipulate XML documents. Both languages borrow from the script concept of using interpreted language for simple tasks. In practice, you could use either language to achieve a given result. One is not more powerful than the other. And yet, each language has a distinct personality. I suspect that, depending on the task at hand, and maybe depending on your personality, you might be more at ease with one or the other, so it is worth learning about them.