Crown Prince Mohammed bin Nayef, the kingdom's 55-year-old interior minister, is a known quantity in Washington, a longtime security chief praised for efforts against Al-Qaeda that saw the jihadist group try to assassinate him.
But Deputy Crown Prince Mohammed bin Salman is a virtual unknown, a favoured son of King Salman elevated to the defence ministry after his father assumed the throne earlier this year. Even Mohammed bin Salman's exact age has not been disclosed. Some estimates put him in his early 30s.
King Salman's pull-out of the two-day summit of Gulf leaders in Washington and at Camp David has prompted widespread speculation.
Some called it a snub connected with Saudi concerns over a nuclear deal with Iran.
But analysts said Salman could not have sent more powerful men in his stead.
Their presence at the summit is "as good as if the king was attending personally," said Jamal Khashoggi, an analyst and head of the Alarab News Channel who has close links with the royal family.
And even if Salman had attended, the two princes would "probably do most of the heavy lifting," a Western diplomatic source said.
Mohammed bin Nayef is in charge of the kingdom's security matters while Mohammed bin Salman heads defence issues, including oversight of the Saudi-led military campaign against Iran-backed rebels in Yemen.
"I think they're obviously viewed as the rising stars" in the Saudi hierarchy, said Frederic Wehrey of the Middle East Programme at the Carnegie Endowment for International Peace in Washington.
Both princes were promoted by King Salman, 79, to the positions of heir and second-in-line to the throne late last month, although they already held powerful posts after Salman was crowned following the death of King Abdullah in January.
- 'Trying to earn his stripes' -
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Mohammed bin Nayef studied politics in the United States and had several military training courses, including under the Central Intelligence Agency, according to experts on the Saudi royal family.
He spent years in the interior ministry, personally overseeing the kingdom's crackdown on Al-Qaeda which waged a campaign of shootings and bombings that killed foreigners and Saudi security personnel between 2003 and 2007.
Mohammed bin Nayef himself became a target, narrowly escaping with superficial injuries in 2009 when a suicide bomber blew himself up while pretending to surrender.
The bespectacled Mohammed bin Nayef "is held in high regard" in the United States for his success against Al-Qaeda, according to David Ottaway, a senior scholar at the Wilson Center in Washington.
"He's a known quantity," Wehrey told AFP. "He has strong personal relationships with many in Washington."
That is not yet the case for the dark-bearded Mohammed bin Salman.
According to a biography from the MiSK Foundation, which the prince established for youth development, he had "a professional career of 10 years" before becoming in 2009 a special adviser to his father and later head of his court when Salman became crown prince.
On January 23 -- the day Salman acceded to the throne -- he named his son minister of defence. Mohammed bin Salman's picture went up on billboards around the country alongside images of the king and Mohammed bin Nayef.
Just weeks after taking charge of the kingdom's armed forces, the prince assumed huge responsibility when the Saudi-led coalition on March 26 began its air war against the Yemeni rebels.
Bruce Riedel, a former CIA officer who directs the Brookings Intelligence Project in Washington, wrote that Mohammed bin Salman "has a reputation for being aggressive and ambitious, as might be expected".
Wehrey said Washington is "a bit more leery" of the inexperienced younger prince, who is "trying to earn his stripes" and whose Yemen campaign has "given a lot of people pause" because of civilian casualties and other concerns.