As of now, or in their prime? Prime? I don't even think that's a question. Mariah, hands down. As far as raw talent, not many people stack up to Mariah. She has since lost a lot of her range, though.
I love both, but Mariah sounded much better in her younger days in the early 90's. I'm not too impressed with her now, and Beyoncé's voice just seems to get better and better every time she releases something new. My vote goes to Beyoncé.