Surprise frontrunner Francois Fillon said his project was "more radical", and vowed to implement sweeping changes in the first three months of his term.
Alain Juppe insisted his proposals were "deep and credible" but lacked the "brutality" of Mr Fillon's plans.
It was the final Republicans party debate before Sunday's concluding vote.
The Republicans candidate is widely expected to face far-right National Front (FN) leader Marine Le Pen in a runoff vote in the presidential election next May.
Self-declared supporters of the centre-right values are choosing between the two former prime ministers. It is the party's first such primary vote, modelled on the US system.
Seven candidates were whittled down to two last Sunday, with Mr Fillon taking an unexpectedly decisive lead of 44.1% to Mr Juppe's 28.5%.
Juppe v Fillon: Battle of France's conservatives
Press review: France's 'Mr Nobody' Francois Fillon
Fillon under fire over women's rights
The past week has seen a campaign marked by a bitter row over the nature Mr Fillon's social policies. Mr Juppe provoked a furious reaction when he questioned whether Mr Fillon, a Roman Catholic, could seek to challenge the legality of abortion.
But polling suggests such tactics seem to have backfired on Mr Juppe, once the favourite seen as the safe choice.
A poll of 908 debate viewers by Elabe suggested 71% of conservative respondents found Mr Fillon more convincing, as did 57% of viewers of all political stripes.
"It is true that my project is more radical and perhaps more difficult," said Mr Fillon.
His controversial economic reforms include cutting half a million public sector jobs and scrapping the 35-hour work week, prompting comparisons to the late British prime minister Margaret Thatcher.
Mr Juppe also proposes cutting jobs and liberalising work hours, though to a lesser extent. Both propose roughly similar cuts to public spending of at least â‚¬100bn (Â£85bn; $106bn).
"I've got a plan for the reforms I want to make in the first three months of the presidential term - and I'm convinced if we don't get these changes implemented in the first three months, the French people will feel disheartened, they'll turn away from politics, and then there'll be a greater risk of the extremists winning," Mr Fillon said.
The country was "on the brink of revolt", which could only be averted with such radical action.
Conservative Le Figaro says the debate "eased tensions on the right" and showed the two contenders "have already formed identical visions and almost similar projects" - in particular the need to end the "vicious cycle of spending and debt".
They have "some differences on security, education and migration, but they are minor", concludes its political editor.
But centre-left Le Monde's political correspondent disagrees, seeing "significant differences" on the European Union, Russia and Syria.
While Mr Fillon wants to "respect the sovereignty" of EU member states, Mr Juppe favours a "powerful EU as a future world power".
On Syria, Mr Fillon wants to put France "back in play" by "opening a channel to Damascus", whereas Mr Juppe wants to "boost transatlantic co-operation" and takes a firm line against President Bashar al-Assad, Le Monde says.
Left-wing Liberation writes that the debate between "Fillon the radical and Juppe the reconciler" was "knotty, often tense, but not aggressive".
It thought Mr Juppe managed to score points over his opponent on some issues, but "not enough to trouble the winner of the first round".
BBC Monitoring reports and analyses news from TV, radio, web and print media around the world. You can follow BBC Monitoring on Twitter and Facebook.
Mr Juppe hit back at Mr Fillon, saying "reform should not be a punishment but bring hope".
"The French social model exists, I want to consolidate it... not break it," Mr Juppe said, referring to the welfare system.
Mr Juppe pledged to bring unity and stability to the presidency - whose current Socialist incumbent, Francois Hollande, has seen his popularity drop to historic lows.
He said he would choose a "loyal" PM and 15 "durable" cabinet ministers who would not "change every 18 months".
The two rivals for the centre-right French presidential nomination have clashed over the level of change they promise to bring, in a TV debate.