Katy has more disadvantages imo. 1st of all she is older (if she ages well like J-Lo it's not a problem tho), also songs like Roar won't work for her a few years later and hwn she tries to act edgy or dark she looks funny. Also I dunno if anyone could take someone named Katy Perry that seriously.
But I don't know she is smart imo and will have a long career no matter what, Rihanna is just more ''cool'' for people to like, appeals to a wider audience etc. We gon wait and see