In the spring of 2013, the University Library at the University of Illinois at Chicago was in the unique position of having access to two discovery systems, Summon and WorldCat Local, at the same time. When tasked with choosing between the two systems, librarians undertook a usability study of Summon and WorldCat Local. The goal of this study was two-fold: to test the ease-of-use of each discovery system with an eye toward identifying one tool to retain for the longer term, and to learn about the search behaviors of different types of user groups. Eighteen subjects, consisting of undergraduate students, graduate students and faculty, participated in the study. Participants performed usability tasks using each tool and answered pre-task and post-task questions. While there was no clear preference among study participants for either discovery layer, individual groups did express preferences. Faculty, for example, preferred Summon to WorldCat Local at a rate of five to one. The study findings are explored in detail through an examination of the three major data sets produced by the usability test instrument: results derived from tasks performed by participants as part of the study; themes and trends identified by the investigators within the recorded participant tests; and discovery tool preferences as determined from pre-task and post-task questionnaires administered to study participants. This study has implications for librarians engaged in information literacy instruction; those considering implementing discovery tools, as well as for librarians currently using Summon or WorldCat Local at their libraries.