  We use simulated merger trees of galaxy-cluster halos to study the effect of
the halo concentration distribution on strong lensing and X-ray emission. Its
log-normal shape typically found in simulations favors outliers with high
concentration. Since, at fixed mass, more concentrated halos tend to be more
efficient lenses, the scatter in the concentration increases the strong-lensing
optical depth by $\lesssim50%$. Within cluster samples, mass and concentration
have counteracting effects on strong lensing and X-ray emission because the
concentration decreases for increasing mass. Selecting clusters by
concentration thus has no effect on the lensing cross section. The most
efficiently lensing and hottest clusters are typically the \textit{least}
concentrated in samples with a broad mass range. Among cluster samples with a
narrow mass range, however, the most strongly lensing and X-ray brightest
clusters are typically 10% to 25% more concentrated.
