Here are some details about the two men:
* A former bus driver and trade unionist with Caracas public transport, the burly and mustachioed Maduro, 50, is a staunch Chavez loyalist chosen by him as the preferred successor.
* Maduro entered politics in 2000 as a legislator in the National Assembly, where his combative defense of Chavez's policies made him one of the president's proteges.
* He rose to become president of the legislature, a post later occupied by his wife, Cilia Flores, a lawyer. When Chavez was sent to prison following a failed 1992 coup attempt, Flores led the legal team that won his freedom two years later. Now Venezuela's attorney general, she and Maduro are seen as the "power couple" in government circles.
* As foreign minister for six years from 2006, Maduro was a faithful ambassador of Chavez's views, including critiques of global affairs from a hard left-wing stance. He won plaudits from foreign diplomats, however, for his affable style.
* Chavez named Maduro vice president in October 2012, reveling in his working-class roots. "He was a bus driver. How they mock him, the bourgeoisie," Chavez laughed.
* Maduro became even closer to Chavez after the president's cancer was first diagnosed in mid-2011, often at his side in Havana and giving brief updates in speeches to Venezuelans.
* Hours before Chavez's death, Maduro accused Venezuela's "imperialist" enemies of infecting Chavez with the disease. He also ordered the expulsion of a U.S. Embassy's Air Force attache for allegedly trying to stir up a military plot against the government.
* Maduro's humble roots appeal to Chavez's many poor supporters. Polls last year showed Capriles to be more popular, but the equation has changed since Maduro got Chavez's blessing, and he is viewed as the favorite.
* Chavez's endorsement of Maduro has quieted the ambitions of other powerful Socialist Party figures, such as Diosdado Cabello, who had been widely considered a candidate for the top job in the future. Cabello, a military man with close ties to the armed forces and business, is not as well liked as Maduro among Venezuelans.
* Capriles, 40, is governor of Venezuela's second most populous state, Miranda. The state, which includes part of Caracas, ranges from the huge Petare shantytown to fishing villages and beaches on the Caribbean coast.
* A law graduate, Capriles became Venezuela's youngest legislator at 26, then won the mayoralty of a Caracas municipality before beating Cabello, a die-hard Chavez loyalist, to become Miranda's governor in 2008. He retained the post in December by beating another heavyweight, Elias Jaua, who was Chavez's vice president at the time.
* In the October 7 presidential election, Capriles was the candidate of the Democratic Unity coalition of more than two dozen parties and organizations that make up the bulk of Venezuela's opposition. He lost, but received 44 percent of votes, the opposition's best showing against Chavez.
* In Miranda, the charismatic and energetic governor is known for riding a motorcycle and heading into slums to supervise projects and talk to working-class voters. On the campaign trail before October's presidential election, he visited hundreds of towns and villages, seeking to project an image of energy, youth and attention to grassroots problems.
* Some say Capriles has deliberately cultivated an almost Chavez-like image of being on the street and in constant contact with the poor. While campaigning, he blows kisses and pumps his fist in a Chavez-like, man-of-the-people style.
* Capriles' maternal grandparents, the Radonskis, fled anti-Semitism in Poland at the end of the World War Two and arrived in Venezuela with just a suitcase stuffed with clothes. Two great-grandparents died in the Treblinka concentration camp. "Imagine that some people in the Chavez government are so ignorant, they actually call me a Nazi," he has said.
* His grandparents set up a lucrative cinema business in Venezuela and, through them, Capriles once met legendary Mexican comedian Mario Moreno - best known as "Cantinflas."
* A basketball player and sports lover, Capriles says he relaxes by finding friends for a game or going for a quiet run after dark. He drinks Red Bulls to keep his energy up, and is a regular at half-marathon races in Caracas.
* Like Chavez, Capriles has been jailed. He was imprisoned for four months on charges of fomenting a protest at the Cuban Embassy in 2002, although he says he was mediating. He was acquitted of the charges at trial, although there is chatter in political circles that the charges could one day be revived.
* If he were to lead Venezuela, Capriles says, he would copy Brazil's "modern left" model of economic and social policies. On the campaign trail last year, he sought to appeal to traditional Chavez supporters, and hammered the government on grass-roots problems from pot holes to electricity cuts, rather than engaging in more nebulous debates over socialism versus capitalism.
* Despite his Jewish roots, Capriles is a devout Catholic, who says his faith deepened in jail. He wears a rosary and likes to visit a shrine on Margarita island each year.
* The governor is single. He receives a torrent of marriage offers via Twitter and Facebook. He says he will find a wife and start a family in his own good time.
* Although describing himself as center-left, Capriles belongs to the more conservative Primero Justicia (First Justice) party, which he helped found in 2000. Foes say he is really an "ultra-right" politician in the pocket of Venezuela's pro-U.S. traditional elite, but masquerading as a progressive.
* To try to discredit him, government officials have targeted his wealthy background, association with conservative politicians linked to Venezuela's pre-Chavez rulers, and his role in the Cuban embassy affair.
* If he had won the October election, Capriles would have become Venezuela's youngest president. He often uses the slang of Venezuela's young and frequently wears informal clothes and a baseball cap.
(Reporting by Andrew Cawthorne, Daniel Wallis and Brian Ellsworth; Editing by Kieran Murray and Peter Cooney)