I feel like Rihanna's look is way more high fashion, which is why she's become more of a muse to designers and fashion houses all over the world.
Whereas Britney at her peak managed to be the sexiest woman in the entire world, while still being the All-American girl next door. She managed to look like the prettiest girl in high school, who everyone knew and thought was gorgeous:
And then with a bit of eyeliner and a skimpy outfit, she looked like a hot **** star, and every straight teenage boy's dream:
Whereas Rihanna's beauty is more exotic and unique, which, like i said, is why she seems to be so revered by the fashion industry.
But like many have pointed out, Britney's peak was just that, a peak. It exists in the past. "Rihanna at her peak" doesn't really exist. She's only getting more and more beautiful as she ages. She's grown into her looks. And she's probably also had a nose job and her chin/jaw done since Music of the Sun. But her "peak" beauty-wise has been like 9 years long. She looks just as good now as she did in the SOS video...if not better.