Well, first of all, there is no scenario number 1, when the U.S. withdraws congress will vote to reinstate the sanctions. The Republicans control the house and the Senate so the vote will surely pass. Besides, in America thanks to AIPAC no politician ever lost votes condemning Iran. The European governments for the most part will stay in the deal but with the risk of being fined billions from the U.S. Treasury, the companies themselves will be afraid to deal with Iran. This, in essence, will make the deal worthless.
So why do I say Iran should stay in the deal? Because there is more at stake than just sanctions, the Israeli's are begging for Iran to walk away so they can say Iran is on the way to a Bomb....we must destroy their nuclear capability. Of course by "We" they mean the U.S.A. (Because Israel doesn't have the balls nor capability to go against us in an open attack). This scenario is real since they practically control Washington and the Press. If we stay in the Europeans are at least semi honorable and they will try to live up to the deal. We will continue to at least sell our oil which is our lifeblood. Also, we can demonstrate that the U.S. and not Iran is a lier and a deal breaker. This may come handy in the U.N. if they try to pass sanctions. I hate to say this because I don't like nukes but we must continue to at least hone our knowledge in the nuclear field. In this new world order only force is respected.