The Spice Girls used to spread out their vocals a lot more evenly compared to Girls Aloud do. I think, however, on GA's previous 2 albums that Nadine has taken a back seat as the other girls have been given more to do.
As for the Sugababes, I would say Amelle and Jade right now. I think Heidi's voice is better for the more mellow songs on the album, Amelle takes the uptempo songs and Jade can do both.