Rihanna is more versatile to me. Katy is a one trick pony. Maybe if katy switched over to more serious sounds like circle the drain but eh idk. Plus rihanna is younger.
Rihanna will outlast Katy simply because she's younger. In a few years, Katy is probably gonna get married, have kids, and then a Vegas residency or something. Though I'm sure when it's all said and done, they will both have had respectable, successful, long careers
Katy won't outlast Rihanna.