Janet has better highs ... but at the same time she has worst lows than Britney's discogrpahy.
The only good part of Janet's discography is from "Control - Velvet rope" (4 albums)
Britney has been more cohesive over time with her albums. she doesn't have an album as bad as some Janet's albums .... and her discography is actually getting better if you compare her latter albums with her first ones.
Janet's discography just got worst after 1998
I choose Britney.