Psychological research on subjective evaluation [35], as well as initial experiments using ourselves as subjects, indicated that this pairwise set-up easier to work with than simply presenting a single sentence and asking whether it is memorable or not; the latter requires agreement on an "absolute" criterion for memorability that is very hard to impose consistently, whereas the former simply requires a judgment that one quote is more memorable than another.
memorable quotes differ in other interesting aspects as well, such as sound distributions.