Politicians and civil servants involved in the 1973-74 power-sharing executive have differing views about how that agreement and this one can be compared.
Mr Roy Bradford, a unionist minister in the Brian Faulkner-led executive, said his reaction now was one of "hope rather than euphoric confidence".
He was concerned that the "lesson of Sunningdale has not been learnt - that things should not be expressed in an equivocal and unclear way, allowing everyone to put their own spin on them".
He gave the example of Seamus Mallon and John Taylor putting "entirely contradictory spins" on Strand Two, the former saying it involved executive powers, the latter saying it did not.
He also thought the "ordinary decent citizen will have some difficulty coming to terms with the release of criminal terrorists within two years".
Mr Bradford felt Mr Trimble was in a somewhat stronger position than Mr Faulkner in 1974 in having loyalist support.
He believed, however, the ordinary unionist who would normally support Mr Trimble would feel "grave disquiet" about how the agreement might affect the Union with Britain.
Mr Austin Currie, an SDLP minister in the 1974 executive, thought this agreement would have a better chance of lasting.
"The loyalist paramilitaries who provided the muscle for the Ulster Workers' Council strike then are on-side this time.
"If Mr Paisley decides to go to the top of the hill, he'll go on his own."
The Provisional IRA had also contributed to the power-sharing executive's downfall.
"Brian Faulkner came back from Sunningdale and told the unionist electorate that the agreement would bring peace. Instead the IRA escalated their campaign."
Another weakness had been the constitutional challenge to Sunningdale in the Republic's courts, which would not happen this time.
Sir Kenneth Bloomfield, who was permanent secretary to the 1974 executive, said there were three main differences this time: a much wider spectrum of parties was involved; in 1973, the agreement had been done in "two bites" - the internal dimension first, the external dimension later at Sunningdale - and had been "glued together at a relatively late stage".
The third major difference was that the agreement this time "will be validated by the people if it is carried by a substantial majority".
He said Faulkner had suffered greatly from the accusation that he had reached an agreement over the heads of the people.