Forster, 31, who reached the Boot Camp stages of X Factor in 2009, has been one of the judges' favourite acts throughout the ITV1 talent show's live shows.
Following his rendition of Snow Patrol's 'Run' in last night's (July 24) semi-final, Dawn French branded the singer a "showstopper", while Jason Donovan called his vocal stamina "insane".
Webber claimed that he was "awed" by the "wonderful" performance. He has odds of 1/2 of winning with Paddy Power.
The winner of this evening's show will star alongside Mel C, Chris Moyles and Tim Minchin in Webber's arena tour revival of the rock opera Jesus Christ Superstar.
Rory Taylor is the second-favourite with odds of 13/8, while Roger Wright is the outsider at 9/1.
The Superstar final airs tonight (July 25) at 9pm on ITV1.
Watch our interviews with the Superstar finalists below: