Actually....I do have an answer.
So far, Janet has had the more iconic career....but, she has basically passed her peak, had her mid-aged mainstream comeback and is pretty much done in terms of major hits.
Britney is still fairly young and at least has the chance to surpass Janet in the long run.
So, as of now...Janet has had the more iconic career (by far) -- but Britney's career is a second place that's closing in...if Britney's motivation re-surfaced, she could pass Janet in the long run.