I suspect that Israel's decision will be based on whether they think the mullahs-with-nukes scenario is imminent, and that diplomatic concerns are a considerably lower priority.
I greatly prefer the "power of democracy" approach myself -- that's one reason I'm working with the Iran Freedom Foundation. However, it's quite possible for the pre-emptive strike trigger point to be interpreted differently by the Israelis and the U.S. Administration, particularly in view of the fact that Iran has missiles that can reach Tel Aviv...