  Among all closed-shell species observed in molecular clouds, molecules with
C$_{3v}$ symmetry play a crucial role, as their rotational spectroscopy allows
them to behave as a gas thermometer. In the interstellar medium, methyl cyanide
(CH$_3$CN) is the second most abundant of those (after ammonia, NH$_3$). Its
isomer, methyl isocyanide (CH$_3$NC) is less abundant but has been detected in
many astrophysical sources. In order to assess their absolute and relative
abundances, it is essential to understand their collisional excitation
properties. This paper reports the calculation of rate coefficients for
rotational excitation of CH$_3$CN and CH$_3$NC molecules with He atoms, from
low (5 K) to moderate (100 K) temperatures. We include the first 74 and 66
rotational states of both $para$ and $ortho$ symmetries of CH$_3$CN and
CH$_3$NC, respectively. A propensity for $\Delta j=2$ transitions is observed
in the case of CH$_3$CN-He collisions, whereas in the case of CH$_3$NC-He a
propensity for $\Delta j=1$ is observed for transitions involving low values of
$j$ and at low temperatures, while a propensity for $\Delta j=2$ is observed
for higher values of $j$ and at high temperatures. A comparison of rate
coefficients shows differences up to a factor of 3, depending on temperature
and on the $ortho$/$para$ symmetries for dominant transitions. This confirms
the importance of having specific collisional data for each isomer. We also
examined the effect of these new rates on the CH$_3$CN and CH$_3$NC excitation
in molecular clouds by performing radiative transfer calculations of the
excitation and brightness temperatures for several detected lines.
