A treaty has signed participants who pass legislation pursuant to it. A declaration is just the UN declaring something. You can determine which one equates to quicker application.
There is a Doctrine of state continuity which means if a state signs a treaty, even if there is a major change of government, the state is still bound by its prior treaty obligations.
Who protects the interests of the people when these types of agreements are made? ICCR – International Covenant on Civil and Political Rights. Commits its parties to abide by the civil and political rights of individuals; including among others: the right of self-determination; the prohibition of torture; prohibition of slavery.