As someone who LOVES Janet and her music and would put her as one of the best female artist of all time I honestly am not sure if I could answer this question.
First of all because I haven't even witnessed her prime, so how exactly I (and probably more of 90% people in this thread) am supposed to compare the cultural impact of the 2? By the time I became aware of the music and pop culture Janet's career was in that post Super Bowl phase.
And Beyonce is obviously a huge celebrity and has made cultural impact I could say I have witnessed ( but cultural impact =/= impact in music and performing and that's what people who are defending Janet are saying, cultural impact is about the celebrity status, people mentioning/referencing pop stars in everyday life and etc).
If we are talking impact in music/videos/visuals then Janet wins by a landslide, but if we are talking about cultural relevancy I'm leaning more towards Beyonce simply because to me she feels like she has been more discussed/bigger celebrity than Janet was in her prime (which again I haven't witnessed and have only discovered trough the stuff I read on the internet/other people opinions).
And why are people talking about album sales now?
