Depends how you look at it. Mariah had the whole decade (90s) but Britney had 5 years as a prime.
But Britney was a bigger pop culture/media fixture than Mariah.
My two faves... I'm gonna say Mariah because her lows arent as low and her highs are higher. For example, Mariahs comeback TEOM sold a lot more than Britneys Circus. theyre both legends though and y'all will deal.