Simon Cowell can't decide between Sharon Osbourne or Mel B for a place on the 'X Factor' panel.The music mogul has started negotiations with the pair and he is expected to name one of them as a replacement for Tulisa - who has reportedly been axed from the panel - but it is proving to be a tough choice. A source said: ''Sharon is seriously back in contention, as is Mel - it could go down to the wire.'' Sharon was a panellist on the show from 2004 to 2007 and she recently dined with Simon and also had dinner with his former flame Mezhgan Hussainy on a separate occasion, while Mel B is still thought to be among the frontrunners for the job despite signing up for 'America's Got Talent' in recent weeks. Louis Walsh and Gary Barlow look set to return to the panel, but Nicole Scherzinger's place looks to be in doubt as sources revealed Simon has not been in touch with her even though she won with James Arthur last year. An insider added to The Sun newspaper: ''It is about getting the right chemistry on each panel and the best dynamic. Simon's focus is getting the American 'X Factor' right then he'll sort out the British show. ''While Louis and Gary are pretty certain to return, it is still all to play for for the women.''