* Winner will be centre-left candidate for prime minister
* Two candidates' ages are 24 years apart
* Voters asked to choose between experience and change
ROME, Dec 2 (Reuters) - The two finalists in a primary to choose the centre-left candidate for prime minister in next year's Italian elections face judgement day on Sunday in a run-off primary after a bitter campaign.
The contest will decide whether Pier Luigi Bersani, 61, or Matteo Renzi, 37, stand in national elections early next year against a still-to-be-chosen centre-right candidate to take over from Prime Minister Mario Monti.
Most polls say the slow-speaking, bald, professorial Bersani will defeat Renzi, who bounces around platforms at rallies in open shirts and jeans.
While markets are wary of Bersani's alliance with a party called Left, Ecology and Freedom, both men have pledged to continue budget discipline started by Monti. They would put more emphasis on economic growth and easing burdens on workers and pensioners.
Bersani, who says he represents experience, won 44.9 percent of the vote in a first-round last Sunday. Renzi, who paints himself as a Kennedy-esque reformer and insists the Democratic Party (PD) needs a big shakeup, got 35.5 percent. There were three other candidates.
A poll by the SWG organisation said Bersani, who is PD leader, would get 53-57 percent in the run-off and Renzi, mayor of Florence, 43-47 percent.
"I don't ask you to like me. I ask you to believe me," Bersani told supporters at a rally on Saturday night, repeating in his stump speech that a steady, experienced hand was what Italy needed in tough financial times.
"A lot of people, and not only in Europe, are watching what happens tomorrow," Bersani told the rally.
The past week has seen bitter argument between the two candidates over whether Renzi violated contest rules by taking out privately-funded advertising urging those who did not participate in the first round to vote for him in the run-off.
Bersani tried to put the spat behind them, saying the party did not need to inflict "friendly fire" on itself.
"As mayor of Florence, I cut costs, I eliminated office cars for city employees," Renzi told a rally on Saturday night in his trademark style, wandering across the stage with a long-lead microphone.
"As prime minister, I will do the rest," he said.
Renzi accused the older generation of the Democratic Party of failing to present a credible alternative, allowing former Prime Minister Silvio Berlusconi's centre right to govern for so long.
"If the other side wins, nothing will change. If we win tomorrow night, there will be a new Italy," he said.
Monti, favourite of the business community, has insisted that he will not be a candidate next year but has said he will come back if the election does not provide a clear winner.
Another posible future role for him is as president of the republic and guarantor that austerity reforms agreed with Italy's European partners continue.
Italy's gross public debt is equivalent to 126 percent of national output, according to the IMF.
Berlusconi's scandal-plagued right, forced from government by the financial crisis a year ago, is in disarray.
Berlusconi said on Monday he would wait to see who wins the centre-left primary before deciding whether to run himself. He has repeatedly changed his mind in the last few weeks on whether to do so.