Just to be fair and unbiased, they have different strengths in dancing and artistry.
Britney has a trained background in ballet and other forms of dancing that no one in pop has touched so far, not even Madonna, Janet, J.Lo, Beyoncé, Shakira, Mýa or Ciara had the varied choreography she did in between the many videos she did from "...Baby One More Time" to "Toxic". They would not have given her heavyweights like Brian Friedman, Kevin Tancharoen and Wade Robson to choreograph her videos if she can't dance. That is why it disappoints many why she does not perform that way since 2007, given that she had the innate skills she exhibited throughout her peak. Of course, her music is limited to dance/pop.
Janet, on the other hand, as an artist, discussed many issues, and of course, she explored more with her sounds. As a dancer, her technique is very intricate but was more urban contemporary and jack swing-inspired, but obviously, what set her apart from the many singing dancers during that time is that she had that fierceness, lasting presence and creativity as seen in the moves and choreography she and Paula Abdul, Tina Landon, Gil Duldulao (among many others) pioneered in the videos she created during her reign.
There is a reason why there was enough room for both artists to become very popular during their career highs.