I don't really like either of them, because their music is not really my style.
I think Christina has a great voice, but I really don't like the style in which she sings (her songs sound "over-sung" to me and lots of other people). I do like that she stands up for herself and what she believes in, even if I don't always agree.
As for Britney, I think she also has a good voice, but only in the songs that suit her. I think her voice sounds great in "Toxic" and similar songs, but annoying in others, like "Not A Girl, Not Yet a Woman". I don't know if she could be really successful in music again, but it is good that she puts her family first.