The Silvretta 400/404/500/505/555 all lack a lateral release at the
toe (instead placing it at the heel). No recent alpine downhill
binding has ever lacked a lateral toe release. This strikes me as a
significant Silvretta disadvantage.
Silvretta advantages? The 500 is a few ounces lighter than the Diamir
III, but lacks a true step-in heel. The 505 has a step-in heel, but
is almost exactly the same weight as the DIII.
So other than climbing boot compatibility, I can’t think of any
reasons to buy a Silvretta.