I think it's very close, they are both heavily influential in the same Genre. You get many Vocalist and entertainers looking upto Janet, but you also get many vocalists and entertainers looking upto Mimi aswell, I'll vote for my fave and be a biased bitch though
Mariah is one of the most prasied female vocalist ever, while Janet strength lies in dancing, performing, music videos and etc, so it depends in what kind of popstars you're comparing them to.
Janet, through her talent and sexuality, created a unique image that urban and pop acts have been drawing inspiration from for three decades, as well as contributing to bridging the gap between pop and urban music with the sounds of New Jackswing, socially conscious lyrics and peerless performance ability, becoming one of the MAJOR influences in performers of all kinds of music today. In addition, she was one of the FEW African-American artists of her time to gain prominence and find major success on the level or exceeding that of her white peers.
Mariah Carey's sound and vocal style influenced generations of R&B and pop singers for decades, and she was the first pop diva to find huge success with hip-hop incorporated into her music, and set a trend alight.
I won't choose. 
