  We analyse the spectra of black hole (BH) and neutron star (NS) X-ray
binaries (XBs) in the hard state using archival RXTE observations. We find that
there is a clear dichotomy in the strength of Comptonisation between NS and BH
sources, as measured by both the Compton y-parameter and amplification factor
A, with distinct groups of BH and NS XBs separated at y~0.9 and A~3. The
electron temperature kTe can occupy a broad range in BH systems, from
kTe~30-200 keV, whereas for NSs kTe is peaked at ~15-25 keV, but can extend to
higher values. The difference between BHs and NSs in y implies that kTe is
higher at a given optical depth for BH XBs. Our results also imply that for NS
systems the accreting material loses ~1/2-2/3 of its energy through
Comptonisation in the corona. The remaining energy is released on the surface
of the neutron star, making it a powerful source of soft radiation, which
alters the properties of the Comptonising corona. Finally, we find evidence at
the 2.4 sigma confidence level that Comptonisation parameters may be correlated
with the neutron star spin, whereas no correlation with the BH spin is found.
Our results highlight a further observational distinction between BH and NS XBs
that is a consequence of NSs possessing a physical surface.
