Some of us know

Firstly, I'm a fan of both ladies. However, I feel that sadly one does outperform the other. Mariah's performance seems to reflect the vocal problems she has had of late. Her vocal quality gets lost in the amount of air she expels in ratio to the amont of tone. Airiness is a dynamic tool, but only when it's done tastefully. She's airy at random times throughout the song, but only to seemingly cover the fact that her voice is not as powerful as it used to be (Visions of Love), and especially in comparison to Whitney's. Mariah's vocal riffs aren't as distinct as they have been, which also shows she just hasn't been exercising her pipes as much as she has. Only at certain points when both ladies are harmonizing, do we hear some of the old Mariah, which sounds a lot like Whitney during her early soprano years, I must say.
Whitney, on the other hand, is pure fire. She finesses with due restraint at the beginning of the song and builds as the song moves toward its climax. Her voice has seasoned over the years and although she takes the alto harmony, she still shows that she can blow at the top of her range. She sings with all of the feeling she's got, while Mariah seems to get caught up in the technicalities of singing and fails to connect with her emotion. That is why in the post production phase, Whitney's voice is allowed to take precedence. She takes the emotional highs of the song even higher.
The song itself could have been better, and probably is better, since it seems to have lost some of its structure when it was edited to fit radio format. However, it serves its function of showcasing both vocalists for who they are at this point in their careers.