Beyoncé has the work ethic, vocals, performance and attitude that stand her miles ahead of all her competitors, the woman is near perfect, I love her music, have done since DC days, a definite legend in the making (if not already).
BUT
If I was asked to choose, I'd favour Britney. The X Factor Simon Cowell and co are looking for, is best described simply as "Britney." Whatever it actually is, she has it in spades. She became almost an instant icon/legend, for a couple of years, she was possibly at her most notorious, without dropping a single record, people are drawn to her. In comparison to Beyoncé her vocals aren't up to scratch, nor is her attitude or performance anymore, but with Britney, she somehow manages to pull it off. She got a genuine standing ovation at the MTV awards a couple of years ago, just for being there and being alive basically.
She's been through a lot, she's shown the harsh reality of showbusiness but also that it's not all bad, there is always a chance for a comeback.
I think in decades to come, when people think back over the past 10 years or so, Britney will be the one that's remembered first, whilst Beyoncé is a bit of an after thought. Not to take anything away from her, as I said I love her, I've seen them both "Live" ("" for Britney) but I see the comparison like MJ and Prince, both big stars around the same time, MJ is the one that is remembered as iconic instantly, but given a bit of thought you remember just how good Prince was as well.