The difference is that Israel in all those years has maintained a policy of absolute restrain, re. its nuclear weapon. It is only used as deterrence. Ahmadinejad however, is constantly threatening to wipe Israel off the map of the earth. That sounds like he wants to use it as an offensive weapon, G-d forbid.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: IDF holds drill simulating rocket attack on central Israel