Yet, mirroring the many splits in American Hispanics' own communities, that is where the similarities end.
Rubio is a darling of the right wing of the Republican Party and beats the drum for slashing government spending. He is staunchly anti-abortion and has said that gay marriage goes against his Christian faith.
Castro has served as the grand marshal in his city's gay rights parade and praises the role of government in society. Each man represents a radically different vision of what his party thinks will appeal to Hispanic voters.
"Both parties need to show that they are open and attractive to Hispanic voters, but they are not at all a unified bloc," said Professor Scott McLeanof Quinnipiac University.
So far it is a battle that Castro and the Democrats have been winning.
In 2008, Latino voters went for Obama by almost 70 per cent. Current polling - though showing a drop in support - still has the Democrat ahead of Romney by 63 per cent to 28 per cent. In the recent Charlotte party convention, Castro was given the all-important keynote address - the first Hispanic to fill that spot - but other Latino party bigwigs were given prominence too. Much was made of Obama's appointment of the first Hispanic woman to the Supreme Court, and the President was also lauded for moves to lift the threat of deportation from millions of illegal immigrants brought to the US as children.
It is that thorny issue of illegal immigration that is the Democrats' real strength. Though Obama is hardly liberal on the issue, the Republican Party has moved far to the right, embracing controversial measures against illegal immigration in states such as Arizona.
Yet one study has estimated that 9.5 million people - the vast majority Latino - live in "mixed" households where at least one person is illegal.
Yet Republican strategists believe they have an ace up their sleeves in the party's embrace of conservative social values that are often shared by many Hispanics, who are often deeply Roman Catholic and family-oriented.
Rubio and other leading Republicans, such as Texan Senate candidate Ted Cruz and New Mexico Governor Susana Martinez, frequently tout their social values. But it has not been a winner overall, because Latino social conservatism often goes hand-in-hand with a belief in government welfare programmes as a way out of poverty. This is especially true as Republicans tout big cuts to healthcare, education grants and food stamps.
Many experts believe the Republican Party faces disaster in the future as Hispanics become an ever more vital part of the electorate. In 2008, about 10.2 million voted, a whopping 25 per cent increase on 2004.
Hispanics represent more than half of all US population growth over the past decade. Due to a youthful population profile, they already represent one in four Americans under 18.
One of the fastest areas of Latino population growth is the Republican heartland of the south.
The battle is already on for Texas - a solidly Republican state that is home to Castro and Cruz. It is noticeable that Texas' Republican Governor, Rick Perry, is already softer on aspects of illegal immigration policy than the Republican mainstream. But if the Republicans are to compete nationally as the voice of Hispanics, the party itself will have to change.
Julian Castro
* Age 37
* San Antonio Mayor since June 2009
* Previously councillor, lawyer
* Democrat
* From Mexican immigrant family
* Married with one child
* Catholic
* Educated at Harvard Law School, Stanford University
Marco Rubio
* Age 41
* Florida Senator since January 2011
* Previously member of the Florida State House of Representatives
* Republican
* From Cuban immigrant family
* Married with four children
* Catholic
* Educated at Miami School of Law, University of Florida
DEMOCRACY AND DEMOGRAPHICS
Women voters
It seems almost ridiculous to call more than half of the electorate a single bloc of voters, but that does not stop the media or politicians of both stripes doing just that as the first lady, Michelle Obama, and Mitt Romney's wife, Ann, do battle.
Roman Catholic voters
You have to look down the ticket to find the battle here, but both vice-presidential candidates, Joe Biden and Paul Ryan, are staunch Catholics. That helps to tick off another demographic for each party.
Celebrities
Clint Eastwood stepped into the breach for the Republicans, giving one of the most memorable speeches in convention history in Tampa - though it got a critical panning for being rambling. For the Democrats, the more youthful figures of Scarlett Johansson and Eva Longoria stepped up to the mic.
Black voters
Okay, this one is not much of a contest. With history-making Barack Obama as the face of the Democratic Party, one recent poll showed Romney's support among black Americans had hit zero. However, Republicans did use their Tampa convention to highlight black conservatives such as up and coming Utah politician Mia Love.
- Observer
