Neither of them are cultural icons. They're both pop culture icons, which is different. But yeah, Beyoncé is a bit bigger pop icon than Britney is. I wouldn't have said that in 2004, when Britney was the biggest pop star on earth, but Britney downgraded herself and her image (or people around her forced her to it) while Beyoncé has kept getting new titles like 'Best performer of our generation', 'Reigning Queen of Pop' etc.