Viewers will see them locked in a war of words in the boardroom as they battle to win a £250,000 investment from Lord Sugar.
James reckons he should get the cash for his IT recruitment firm despite it being his first venture.
While Sarah wants victory to boost her confectionary business after seven years.
But she has come under fire for not expanding its website.
As Lord Sugar quizzes them on their business plans they rip into each other.
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Sarah says: “I’ve grown that business on my own.
“I’ve got those sales in, £430,000 while working…while being a full-time mum.”
But James hits back: “If my business was going for seven years I’d have scaled it a lot quicker.
“My investment will be spent on people.
“These people will be earning money from day one…I’m hungry for this.
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“This is everything I know.”
Sarah takes a swipe at James by saying: “I think, Lord Sugar, if I was looking at us as two candidates, it’s the difference between an employee and an entrepreneur.”
A furious James says: “A entrepreneur with £400,000 turnover…”
But Sarah hits back: “Well, it’s £400,000 more than you have James.”
He then tells her: “But you don’t take risks.”
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Sarah, 35, is one of the older candidates in this year’s series, while James is just 26.
In another swipe, she says: “Everything is in place to use this money wisely.
"Not make rookie mistakes…not make naive statements.
“There’s no USPs in the recruitment business, everything about it is naive.”
But James asks: “Then why has your business taken seven years to get to £400,000?
The Apprentice 2015: funniest moments
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“My business would be there a lot, lot quicker.
“We’d be making a lot more money than you in a lot quicker time.”
Fans will find out who wins the investment in tonight’s final, which kicks off at 9pm on BBC One.
This year’s winner will also have Lord Sugar as their business partner.
The Apprentice has been on screen for 13 years but still pulls in bumper ratings.
Tonight’s climax is expected to draw in more than eight million viewers.