Some people seem to forget Teenage Dream was only Katy's 2nd album, whilst Loud was Rihanna's 5TH. I would have been shocked had Rihanna not sold more albums. Comparing both to where they are in their careers Teenage Dream was far more successful.
I like the Loud era more, but Teenage Dream was more successful in the sense that it furthered Katy's career. In Rihanna's case, while it did give her a proper comeback from the possibly career-killing Rated R, we still haven't reached the end of her album sales rollercoaster, as TTT shows. Teenage Dream positions Katy to have another huge era if she gives it a rest soon and takes a break for a year or two, and it made her a much bigger name worldwide.
Everyone knows TTT was rushed. If she took the time to properly set up for the next era instead of releasing the first single WFL 4 weeks after Cheers was TOP 10 . It would've been a different story. LOUD however did further Rihanna's career in the sense of reestablishing her as a Global Star and made her bigger than ever. It also helped WFL become her biggest hit yet. The album roller-coaster issues in Rihanna's fault seeing how she's retarded enough to pops out albums as quick as she changes her hair. While with Katy thought TD was successful, she already targeted a narrow demographic of kids/Teens (primarily) and established her brand as the cotten candy girl. I don't see the dark "era" & more "serious" music transitioning well with her fans.