You drop a hydrocal piece and you almost certainly break it. Zero flexibility. Resin is much more forgiving when mishandled. More flexible.
Resin generally weighs less by volume.
Large hydrocal pieces (say 12 X 24 inches) are best cut very slowly with a hsnd saw to prevent chipping/breakage.
Resin can be easily cut with care on a radial arm saw with a 160-200 tooth blade.
Hydrocal can be easily repaired using spackling paste.
I have used both resin and hydrocal for large building fronts and am happy with both, but my preference would be resin.