As XU95 said, Dayton has historically been Xavier's A-10 rivalry. Ironically, UC has been Xavier's biggest out of conference rivalry, and Xavier fans have had to endure how great the big east has been from our UC friends and neighbors, and now the schools appear to be headed in different directions.
Interestingly, Jay Wright stopped by the Cintas Center yesterday (Xavier's basketball arena) when Villanova was in town yesterday playing UC:
http://cincinnati.com/blogs/xavier/2013 ... as-center/Before Villanova played UC tonight at Fifth Third Arena, head coach Jay Wright and an assistant stopped by Cintas Center to check out out the Musketeers’ digs. He chatted casually (and quickly) with some hoops staffers about the landscape of college basketball and toured the facility. Villanova, of course, is part of the Catholic Seven splitting from the Big East. And Xavier is one of the schools rumored to be on the new conference’s short list. Hmm…checking out a potential future conference mate?