OP, do not be ridiculous. Whitney has a spectacularly beautiful, rich, agile and powerful voice whose registers were perfectly connected. If she lost her upper register relatively soon and her tessitura kept getting lower, it's because she was a heavy smoker and drug user since adolescence, not to mention that she started to continuously sing in the soprano range long before she was physically ready to do so - however, her lower register was strong and full and the column of sound remained unchanged as she rose up through the octaves. Also, her technique was good enough to enable her to sing part of Rigoletto's La donna è mobile without any effort, which impressed even Pavarotti himself. Mariah would NEVER be able to do this.
Yes, Mariah has a brighter voice, a slightly higher tessitura and the uncanny ability to transition from mixed voice to a disconnected whistle register, but vocally she has always had lots of problems. Her low notes are airy and thin, and by 1997 her notes above D5 were already sounding harsh and strained. Mariah also started to constantly sing soprano long before she was ready to do so and if you top that with the fact that she loves to drink and her technique has always been rather uneven, you can see why she has always had so many problems. I have never believed that her classical singer mother really taught her how to sing, because if she had done so her high notes would have always sounded effortless and her registers would have been properly developed.
So yes, Whitney's overall range was more limited, but her voice was better developed and her tone was richer and fuller. In terms of agility and elasticity (doing very difficult things like switching registers several times within the same phrase), Mariah beats her to the punch. Regarding who has the more beautiful voice, that is a matter of personal taste.
Whitney was at her best between E3 and E5, and up to around 1995 could belt up to A5. Using her head voice, she was comfortable up to B5, but everything above G#6 was quite strained. After that, her voice rapidly deteriorated, her tessitura was lowered and everything became a raspy, airy mess.
Mariah's voice is at its best between G3 and E5 - below G3, everything becomes hazy and weak and above E5, her belts have never been relaxed, even if she could go up to G#5 in the past. Her head voice is also remarkably limited: in the past she was comfortable singing up to A5, but after that it became airy up to D6, and then she switched to whistle register. Her whistles are impeccable, though, and she is much more comfortable vocalizing above C6 than Whitney ever was.
So yes, the both of them have similar placements (they are definitely high mezzos), but they ruined their voices with bad habits, lack of discipline when it comes down to technique and singing too high before their vocal chords had properly matured.