Right now, Katy. But that's because she is at every awards show possible and is constantly promoting, whereas Rihanna spends more time doing her promo in Europe it feels. (But that may be just because I'm from the US and I don't feel there is a lot of promo here but am always seeing European promo form her)
Rihanna has 10 #1's (+3 as a featured artist) since 2005. Katy has 9 #1's since 2008. Katy's batting average is better with scoring #1's. On average Katy sells more albums and singles than Rihanna but, in general, Rihanna most likely has more record sales than Katy.