I actually had a little debate about this in law school. "Relevancy," in Black's Law Dictionary (4th edition --old!), is the term under which concepts of evidentiary relevance are discussed. My layman's dictionary, some sort of Webster's, lists "relevancy" and cross-references to "relevance."

In the legal profession, at least, it may be a question of personal preference. My personal preference is to speak like the rest of the public does and use "relevance."