  We present a comparative study of stellar winds in classical supergiant high
mass X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs) based
on the analysis of publicly available out-of-eclipse observations performed
with Suzaku and XMM-Newton. Our data-set includes 55 observations of classical
SgXBs and 21 observations of SFXTs. We found that classical SgXBs are
characterized by a systematically higher absorption and luminosity compared to
the SFXTs, confirming the results of previous works in the literature.
Additionally, we show that the equivalent width of the fluorescence K{\alpha}
iron line in the classical SgXBs is significantly larger than that of the SFXTs
(outside X-ray eclipses). Based on our current understanding of the physics of
accretion in these systems, we conclude that the most likely explanation of
these differences is to be ascribed to the presence of mechanisms inhibiting
accretion for most of the time in the SFXTs and leading to a much less
efficient photoionization of the stellar wind compared to classical SgXBs.We do
not find evidence for the previously reported anti-correlation between the
equivalent width of the fluorescence iron line and the luminosity of SgXBs.
