If I was a woman, I'm sure I'd choose being the best tennis player, given that tennis has traditionally been by far the biggest sport for women. This year is the 50th anniversary of the WTA tour being set-up, and well before then, Suzanne Lenglen's huge popularity was the major reason why Wimbledon moved to a large venue (its current day one) in the 1920s. The 4 grand slams have traditionally been the 4 biggest annual sporting events on the women's side, though I'm not sure if the women's Champions League has caught up there.
Football overtaking tennis to become the undisputed no. 1 sport for women is inevitable though IMO. That will be facilitied by the the increased recognition of star players such as Alex Morgan, Alexia Putellas, Ada Hegerberg etc. including them appearing in FIFA games, the rising popularity, success and money of / in the top women's European leagues and the Champions League season by season, plus of course the women's World Cup and Euro tournaments, alongside the numerous inherent advantages that football has over tennis as a team sport (50% of games at home, significantly less travelling, guaranteed contracts, more expenses being subsided, getting to train, play and socialise with team-mates etc). Football will surely be seen more and more as the better career option for women / girls as well as men / boys.