Israel and the US differ over the timing and level of risk posed by Iran's nuclear efforts. The US sees a mature or serious Iranian nuclear threat as coming well after 2010. Israel claims to see it as coming as early as 2009 -- although much of this may be Israeli hype designed to push the US into diplomatic action, and military action if that fails.
Official US policy is to leave all options on the table, and emphasize diplomatic activity through the EU3 and the UN. The US estimates of timelines for Iran's nuclear and missile efforts also leave at least several years in which to build an international consensus behind sanctions and diplomatic pressure, and a consensus behind military options if diplomacy fails.
The US would also have the potential advantage of finding any Iranian “smoking gun,” improving its targeting and strike options, and being able to strike targets in which Iran had invested much larger assets. The fact Iran can exploit time as a weapon in which to proliferate, doe not mean that the US cannot exploit time as a weapon with which to strike Iran.
Israel, on the other hand, sees Iran as an existential threat. A single strike on Tel Aviv and/or Haifa would raise major questions about Israel's future existence.
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