  We study the branching fractions and the lepton polarization asymmetries of
rare $B \to X_s \tau^+ \tau^-$ and $B \to K^{(*)} \tau^+ \tau^-$ modes within
the Standard Model and in the Appelquist-Cheng-Dobrescu model, which is a New
Physics scenario with a single universal extra dimension. In particular, we
investigate the sensitivity of the observables to the radius $R$ of the
compactified extra-dimension, which is the only new parameter in this New
Physics model. For the exclusive transitions we study the uncertainties due to
the hadronic matrix elements using two set of form factors, finding that in
many cases such uncertainties are small. In particular, we show that the
$\tau^-$ polarization asymmetries become free of hadronic uncertainties if
Large Energy relations for the form factors, valid for low momentum transfer to
the lepton pair, are used. We also consider $K^*$ helicity fractions and their
sensitivity to the compactification parameter.
