I think your a little confused because if Israel used it's nuclear weapons on Iran there will be no more Iran but Israel will survive. That said i would always keep nuclear as a very last option. On the tiny chance there is war it will be with conventional not nuclear anyway.
Ex-FM Lieberman: Firebomb attack in Jerusalem a reminder that norm is Arab terror against Jews (Haar
from the article: U.S. strike on Iran likelier than ever, former CIA chief says