(Updates with voter comments, candidates casting ballots)
SANTIAGO, Dec 17 (Reuters) - Chileans voted in a runoff presidential election on Sunday that will determine if the world's top copper producer stays on its center-left course or joins a tide of Latin American nations turning to the right in recent years.
They are choosing between billionaire former President Sebastian Pinera, 68, a conservative who was considered the front-runner but earned fewer votes than expected in last month's first round, and center-left Senator Alejandro Guillier, a 64-year-old former journalist.
Both candidates would keep in place Chile's longstanding free-market economic model, but Pinera has promised lower taxes to turbocharge growth while Guillier wants the government to press on with outgoing President Michelle Bachelet's overhaul of education, taxes and labor.
The contest comes ahead of a long stretch of elections in Latin America in 2018. While populist candidates on both the right and left are polling near the top in the countries with the largest economies, Brazil and Mexico, none advanced to the second round in Chile.
Still, strong-than-expected support in Chile for hard-left Beatriz Sanchez and far-right Jose Antonio Kast in the Nov. 19 first round pushed Pinera and Guillier to try to attract voters unhappy with the status quo in what is widely considered Latin America's most developed nation.
Large swathes of voters were left with the option of voting for candidates they did not particularly like or staying home in one of the few countries in the region where voting is not mandatory. Many are simply disillusioned by politics.
"The question is, "Who hates the other side more," said University of Chile professor Robert Funk.
Chilean financial markets rallied in the final week of campaigning in the country of 17 million people with a $250 billion economy, boosted by hopes of Pinera's victory, along with firm prices for Chile's main export, copper.
No major polls were made public in the past month, but consultancy Teneo Intelligence said in a report that Pinera, who took more than 36 percent of first-round votes, "is likely to retain a slight edge" in a very tight race.
Guillier, the flagbearer for Bachelet's Nueva Mayoria coalition, garnered just 23 percent of first-round votes, the weakest performance by any center-left candidate since Chile's return to democracy in 1990 and a reflection of broad discontent with Bachelet.
Leftists have complained that Bachelet's social policies are too timid while the conservative opposition says they have stoked market uncertainty and crimped private investment, causing average economic growth to slow to 1.8 percent per year.
HOPE FOR UNITY
The election on Sunday was the first in which Chileans living abroad were allowed to vote, adding another layer of uncertainty.
Pinera called for unity after what has been a bitter campaign and reminded Chileans of the national strength they had shown during the rescue of 33 trapped miners in 2010, the high point of his first term that ended in 2014.
"My great hope is that after this election, despite the division, we can return to unity," he said after voting in Santiago.
Guillier told a crowd in his hometown of Antofagasta, near the country's main copper mines, that he was optimistic about winning by a "marked difference."
To win, Guillier must capture votes from Sanchez supporters, who said she would vote for him to protest Pinera. Construction worker and Sanchez supporter Patricio Flores, 36, said he would follow her lead.
"We can't have a billionaire running our country. Obviously he is going to protect his own interests," Flores said after voting in Santiago.
Guillier, a former radio and television journalist of more modest means, has pledged to increase access to free higher education and write workers' and indigenous rights into a constitution to replace the current dictatorship-era document.
Pinera, a Harvard-trained economist who made his fortune introducing credit cards to Chile in the 1980s, said he would create a public pension fund to compete with Chile's much-criticized private pension funds, and expand free education.
"I voted for Pinera because I am an entrepreneur. I value my own efforts and do not expect much from the government," said Rosario Poma, 53. "I think (Pinera) will be good for investment."
(Additional reporting by Felipe Iturrieta and Antonio de la Jara; Writing by Caroline Stauffer; Editing by Alistair Bell and Jeffrey Benkoe)