  We use the Copernicus Complexio (COCO) high resolution $N$-body simulations
to investigate differences in the properties of small-scale structures in the
standard cold dark matter (CDM) model and in a model with a cutoff in the
initial power spectrum of density fluctuations consistent with both a thermally
produced warm dark matter (WDM) particle or a sterile neutrino with mass 7 keV
and leptogenesis parameter $L_6=8.7$. The latter corresponds to the "coldest"
model with this sterile neutrino mass compatible with the identification of the
recently detected 3.5 keV X-ray line as resulting from particle decay. CDM and
WDM predict very different number densities of subhaloes with mass $\leq
10^9\,h^{-1}\,M_\odot$ although they predict similar, nearly universal,
normalised subhalo radial density distributions. Haloes and subhaloes in both
models have cuspy NFW profiles, but WDM subhaloes below the cutoff scale in the
power spectrum (corresponding to maximum circular velocities
$V_{\mathrm{max}}^{z=0} \leq50~\mathrm{kms}^{-1}$) are less concentrated than
their CDM counterparts. We make predictions for observable properties using the
GALFORM semi-analytic model of galaxy formation. Both models predict Milky Way
satellite luminosity functions consistent with observations, although the WDM
model predicts fewer very faint satellites. This model, however, predicts
slightly more UV bright galaxies at redshift $z>7$ than CDM, but both are
consistent with observations. Gravitational lensing offers the best prospect of
distinguishing between the models.
