american idol is a really good show but will probably not be as popular as the x factor, though they are probably on at different times of the year so it might not matter too much either way.
probably looking at how it is now with american idol on early in the year and x factor on at the end of the year, they are not really going to clash too much.
american x factor will probably end up being the most watched out of the two shows when it gets going, as it will be something new and people will watch just to see how it goes.