As the X Factor contestants go head to head, the judges are placing their bets on who could be the winner for 2008. Tonight's ITV1 show sees the remaining nine acts taking on Disco Week, with a special guest performance by Pop Idol winner Will Young. Meanwhile, X Factor charity single Hero is certain to reach the number one spot and is set to be the fastest selling of 2008. It is outselling the rest of the Top 10 singles combined, according to latest sales reports. The record, in aid of Help The Heroes and the Royal British Legion Poppy Appeal, is also outselling the current No 1 single by Girls Aloud , The Promise, by four to one. All 12 finalists in this year's series, including Manchester's now eliminated Scott Bruton, recorded the cover of Mariah Carey's classic. HMV spokesman Gennaro Castaldo said earlier this week: "This is proving to be one of the biggest single releases for years if not the decade. "Only Leona Lewis's debut hit A Moment Like This, which sold over 500,000 copies in its first week, can compare. "It's selling more than most Christmas Number Ones would, and HMV has placed an urgent order for more copies.