  We report Electronic Raman Scattering measurements on optimally doped Zn and
Ni substituted YBCO in A$_{1g}$ and B$_{1g}$ channels. We show that the
superconducting gap energy is independant of magnetic Ni and non-magnetic Zn
substitutions. On the contrary the collective A$_{1g}$ mode follows the
critical temperature $T_c$ with two distinct slopes for Ni and Zn, and tracks
the magnetic resonance seen in inelastic neutron scattering. We explain the
unconventionnal energy dependence of the superconducting gap and discuss the
behaviour of the A$_{1g}$ mode within magnetic and non-magnetic impurities.
