  On the seven dimensional Euclidean sphere $\mathbb{S}^7$ we compare two
subriemannian structures with regards to various geometric and analytical
properties. The first structure is called trivializable and the underlying
distribution $\mathcal{H}_T$ is induced by a Clifford module structure of
$\mathbb{R}^8$. More precisely, $\mathcal{H}_T$ is rank $4$, bracket generating
of step two and generated by globally defined vector fields. The distribution
$\mathcal{H}_{Q}$ of the second structure is of rank 4 and step two as well and
obtained as the horizontal distribution in the quaternionic Hopf fibration
$\mathbb{S}^3\hookrightarrow\mathbb{S}^7\rightarrow\mathbb{S}^4$. Answering a
question in arXiv:0901.1406 we first show that $\mathcal{H}_{Q}$ does not admit
a global nowhere vanishing smooth section. In both cases we determine the Popp
measures, the intrinsic sublaplacians $\Delta_{sub}^T$ and $\Delta_{sub}^{Q}$
and the nilpotent approximations. We conclude that both subriemannian
structures are not locally isometric and we discuss properties of the isometry
group. By determining the first heat invariant of the sublaplacians it is shown
that both structures are also not isospectral in the subriemannian sense.
