Asking who is more likely to do something stupid is a little tricky. For my part, I have trouble calling an attack by Israel on an extremely unstable neighbor who is attempting to create an atomic weapon and conjointly calling for the eradication of Israel, “stupid” per se, but I suppose if we are simply referring to the plausibility of the sparking of a massive war, using “stupid” might sort of work. However, I think the possibility of either of them taking either course of action is relevant to Iran’s nuclear arms progress.
I have no doubt that if Israel’s big bro, the US, suddenly disappeared tomorrow, and Iran happened to, at the same moment, develop a couple WMD’s, Iran would immediately attempt to wipe Israel off the face of the planet. However, I’m going to give them the benefit of the doubt that policy makers in Iran aren’t willing to trade the destruction of Israel for their own political sovereignty, which would be eradicated by NATO, and more likely, the U.S, should they ever become so bold. Though…it would seem that giving middle eastern regimes the benefit of the doubt in regards to the brute military realities they face as global little guys might be over generous, as Saddam so eloquently illustrated.
But in all honesty, I can’t entirely see it being reasonable for Iran to consider actually nuking Israel a decision on the table. Having said that, I would still defer to Israel’s judgement, if they decide to attack Iran. Israel is the one that has everything to lose should Iran decide they want to take down a million Jews. Should my judgement of Iran be proper, then even after an attack on nuclear facilities, Iran would still, not good humor of course, but nonetheless, be hesitant in getting involved in a war where, should Israel be faced with the prospect of defeat, the US would come sweeping in for the rescue.
So I suppose to answer your question, Israel (with the caveat that Iran bears the nature I have assumed) is undoubtedly more likely to launch any sort of attack, or make any sort of “stupid,” decisions.