I'll put it like this. Janet created waves, Mariah just rode waves. Mariah was nothing but a hit maker. She didn't really break barriers, grounds, or do anything ingenious or new.
I love both ladies, but Janet has impacted pop culture more. It's sad when ATRL refuses to see Janet's impact.
She pretty much revolutionized a new standard for music video and performances. Her influence has no bounds. Most female pop stars credit her as an influence. And even then, it's just not females, but males too.
Mariah Carey's impact/influence is more voice oriented..where as Janet is an overall entertainer and performer that gives her the edge.