I think there's a couple of ways to answer the question.
If you're asking who is more popular with the general public (everywhere outside of
ireland anyway), it's undoubtedly Rousey. She's the one who's all over mainstream media and in movies and everything.
If we're talking with just fans of the sport... actually it's probably still Rousey in terms of "popularity", because fans seem to be split between loving and hating McGregor. I'd say McGregor would probably be a slightly bigger draw with the fans, purely on the basis that all fans pretty much know that a Rousey fight will be a squash match, whereas McGregor brings a lot of unanswered questions.