Based on your personal experience. For example, I know almost every Britney single even from 15 years ago and I don't know how. They must have spun it on the radios here like crazy or something. I know just a few of Bey songs. Is it the same with you and your place?
Tbh Bey is pretty famous/successful here in Brazil, but people definitely remember Britney's hits, they get played on radio quite frequently. I'd say they are tied, but I'm obviously voting Britney.
If I ask for Beyoncé singles here in Latin America the GP would say Crazy in Love, "the song with Shakira", Single Ladies, Halo, Run The World and Drunk In Love.
Well I live in a very urban radio dominated area, so literally everyone knows Beyoncé, even her flops like Get Me Bodied and Partition were major cultural phenomenons.
I think people in New York City remember more of Beyonce's singles. The only recurrent song I hear on the radio around here by Britney is Toxic. I can't speak for the rest of the U.S. though.