The whole "Who is more popular, Sydney or Femke?" is just stupid, but I will say this.
Femke Bol has 26 wins on the Diamond League circuit. Twenty-six! That is among the all-time highest victory totals. She has competed in almost every different venue and been viewed by a lot of track fans in many different countries/regions. She is very likeable and her amazing comeback in the Mixed 4x400 in Paris, getting the baton down about 20 meters, in 4th place with 150m to go, and pull out the victory, was one of the biggest moments of the entire Olympics in any sport.