Boothroyd says he has the better side after finishing third in the league but still claims victory for Watford in Cardiff will be a shock of biblical proportions.
"We're the little David that has to go there and do what he did to Goliath," he said in The Mirror. "The head says Leeds, the heart says Watford.
"I would imagine everyone would want Watford to win, unless they're Leeds fans. I genuinely believe we are the underdogs. Look at size of grounds, the expectation, the tradition of Leeds.
"But ours is a terrific story, the chance to go from 18th in the Championship to the Premiership the year after. It is great romance."
However, Sir Elton John won't be there to cheer on his beloved team but he expects 22-goal leading scorer Marlon King to illuminate the occasion.
Former chairman Elton, who also missed Watford's 1999 Wembley Play-off win over Bolton, has a prior charity commitment.