  M-dwarfs are emerging in the literature as promising targets for detecting
low-mass, Earth-like planets. An important step in this process is to determine
the stellar parameters of the M-dwarf host star as accurately as possible.
Different well-tested stellar model atmosphere simulations from different
groups are widely applied to undertake this task. This paper provides a
comparison of different model atmosphere families to allow a better estimate of
systematic errors on host-star stellar parameter introduced by the use of one
specific model atmosphere family only. We present a comparison of the ATLAS9,
MARCS, Phoenix and Drift-Phoenix model atmosphere families including the
M-dwarf parameter space (T$_{\rm eff}=2500$K$\,...\,$4000K,
log(g)=3.0$\,...\,$5.0, [M/H]=$-2.5\,...\,0.5$). We examine the differences in
the (T$_{\rm gas}$, p$_{\rm gas}$)-structures, in synthetic photometric fluxes
and in colour indices. Model atmospheres results for higher log(g) deviate
considerably less between different models families than those for lower log(g)
for all T$_{\rm eff}=2500$K$\,...\,$4000K examined. We compiled the broad-band
synthetic photometric fluxes for all available model atmospheres (incl.
M-dwarfs and brown dwarfs) for the UKIRT WFCAM ZYJHK, 2MASS JHKs and Johnson
UBVRI filters, and calculated related colour indices. Synthetic colours in the
IR wavelengths diverge by no more than 0.15 dex amongst all model families. For
all spectral bands considered, model discrepancies in colour diminish for
higher T$_{\rm eff}$ atmosphere simulations. We notice differences in synthetic
colours between all model families and observed example data (incl. Kepler 42
and GJ1214).
