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 1TeV, and favors a value
of α3(mZ) 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.Comment: 18 pages,LAEFF-93/014,REVTEX-2.1, 5 figures not included, available
upon request (include FAX number)