The mediators declare both alternatives - the construction of a new airport on a site currently occupied by the project's opponents or the extension of the runways at the already-existing Nantes-Atlantiques airport - "reasonable options" in a report submitted to the government on Wednesday.
But they both have disadvantages and there is no "perfect solution", they point out.
Building a new airport would mean more urban sprawl and damage an environmentally sensitive site, as the protesters and farmers who have occupied the site since 2009 point out, the report concludes.
It would also cost 730 million euros, compared to between 365 million and 460 million euros for the extension, the mediators estimate, although compensation for the cancellation of contracts with the BTP Vinci group are not included in the latter figure.
But the mediators also concede that extending the Nantes-Atlantiques airport runway would mean "significant" noise pollution for residents of the surrounding areas and call for an "urgent" revision of the present anti-noise measures.
Five million passengers pass through Nantes-Atlantiques every year at the moment and the figure is expected to rise to nine million by 2040.
Opposition predicts "capitulation"
On receiving the report, Prime Minister Edouard Philippe promised a "clear decision", which would mean a "return to normal" so far as public order is concerned, before the end of January.
His words echoed a similar pledge by Macron in an interview with Le Monde newspaper published on Tuesday.
Even before a decision has been announced, members of the right-wing opposition Republicans were predicting an "unprecedented capitulation" to the protesters.
"Who makes the decisions in France?" Republicans spokesman Guillaume Peltier asked on Cnews TV. "Two-hundred zadistes [site-occupying] activists? The law of the strongest? Anarchy? The rule of law and the authority of the republic?"
He called for the site to be evacuated, by the armed forces if necessary.
Peltier also cast doubts on the objectivity of the report, pointing out that two of the three mediators are close to Environment Minister Nicolas Hulot, a veteran green campaigner.
Long battle in courts and on ground
Five legal cases have been filed against the project and supporters of a new airport have promised to take up the legal cudgels if the government decides against them.
They also point to a local referendum, held in 2016, which showed 55 percent of those voting in favour of the Notre-Dame-des-Landes plan.
Green party member Cécile Duflot, who was housing minister in the previous government, on Wednesday claimed that then-president François Hollande told her the Notre Dames des Landes plan was "not a good idea" but that there was a "problem with Jean-Marc".
That was a reference to his prime minister Jean-Marc Ayrault, a former mayor of Nantes who was a fervent advocate of building a new airport.
Ayrault was still defending the idea on Wednesday, insisting that, although it was first mooted in 1970, "This is not a project from the past".