They are about the same but I think Pink is quite a bit better. To me there is something much more appealing about Pinks songs compared to Christinas anyway.
I love the both of them but I'll have to go with Pink because even though they both can sing Pink has much more self control and Christina overdoes it, she oversings too much.
I don't like either of them's music (is that gramatically correct?), but Christina Aguilera is rather hot while Pink looks like a dude from a distance.