  In this paper we study and compare susy unification using two different
approaches in order to take into account the effect of light particle
thresholds on the evolution of gauge couplings: the step--function
approximation, on the one hand, and a mass dependent procedure, which gives a
more accurate description of the dependence of the results on the masses, on
the other. We also include the effect of heavy thresholds, when $SU(5)$ is
chosen as the unifying group. We find that the mass--dependent procedure
excludes scenarios where all susy masses are below $1\;TeV$, and favors a value
of $\alpha_3(m_Z)$ near its upper experimental bound, contrary to the results
obtained with the step--function approximation. We underline the dependence of
the results on the procedure chosen to deal with light thresholds.
