No Whitney was the better choice. I think had Madonna got the role she would have wanted it to become softcore porn and Kevin Costner knew that. Madonna was to hardcore ,sex everything during this period. And I don't think she would have been believable as romantic lead.
Kevin Costner said in an interview years ago that he and director Mick Jackson had both had Whitney in mind from the start. The Bodyguard was a movie that had originally been proposed in the 1970s. Originally cast were Diana Ross and Steve McQueen, then it changed to Diana Ross and Ryan O'Neal. The movie had fell through and the movie was proposed again years later with the idea of Whitney and Kevin. Whitney had said she wanted to break into movies and that the part of Rachel mirrored her completely. The only problem was Whitney's obligations to touring for her "I'm Your Baby Tonight" album (released in 1990) After Whitney finished the tour she began shooting on The Bodyguard.
The point is that Madonna was never a possibility for the movie. The premise was a black woman and a white man fall in love while he's protecting her. Last time I looked Madonna wasn't black.