More likely a combination of both: Mossad for the details of what is inside Iran's program and deploying the worm, and the US for the technical skills to build it.
Israeli software skills are excellent. Of all the assets the US could provide, that's one Israel doesn't need and likely wouldn't accept. The US could provide electronic surveillance: locations of people, details of local police operations, satellite photos, etc.
While technically it can be done by anyone in the world, it takes certain thriving technology culture to ensure that enough smart people penetrate the ranks of employees in government bureaucracies. Israel's overall IT scene is non-remarkable, not any more than one of Sweden or Greece.
The USA on the other hand, has decades-long successful track record of industrial sabotage involving computers, and NSA alone probably employs more talent than Mossad could ever afford to.
Israel doesn't just recruit from Israel. They recruit from Jews internationally.
This is a major part of their strength. How many brilliant Jewish software people live in the USA? Those are the people Israel would recruit, along with their homegrown talent.
Having said that I agree that the NSA would be more qualified than Israel at creating Stuxnet.
