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The differences are so small, that if one show decided to change just a few things, it would turn into the other show. In the end, the viewers of XF decide the bottom 2 and ultimately the winner anyway, so I don't think the audience screws up talents more in AI than XF.

As for winners I don't know much about the British ones, but like you said Kelly Clarkson made it big, Carrie is a huge star in the US at least, the biggest music market in the world, David Cook is doing well and Jordin Sparks had a couple of overseas hits as far as I remember.

X Factor had Leona Lewis, but biggest star of this era? I don't know. Then you mention the quality of the singers participating, that's highly subjective.

I guess I'll say AI because I know more people from that, but they're so alike I'd vote for both if that was an option.