The latter is Bush's policy to date, and by the time he gets through another 12 years of dickering with the UN it will be too late. I don't see much difference between him and Kerry on this issue, although Kerry wants to add yet another layer of dickering with his bizarre plan of first "proving" that Iran has bad intentions by offering them uranium on the condition that they return the spent fuel (when they decline this offer, we'll know for sure that they're bad guys).
The current neo-con line on this is absurd - the claim is that we can't be sure we can take out all the sites (there may be as many as *gasp* 15), so we'd better sit back and twiddle our thumbs while waiting prayerfully for the IAEA and the UN to defend us from this threat. This is a thin smokescreen for the idea that America's self-defense must be balanced with other considerations, such as avoiding killing "innocent" Iranians, meeting the global test (and here Bush is truly a case of the pot calling the kettle black), and above all, not appearing to act too selfishly.