Neither

From a technical point of view Mariah (in her prime) is a far more gifted vocalist. I appreciate Aguilera but her technique is bad and she is declining. I don't think I've heard any belted vibrato from her in years. Obviously the sound of someone's voice is down to personal preference but when it comes to actual technical vocals, Mariah was better. This doesn't take away from the fact that she comes across as a bit of a knob and also can't sing half as well as she used to.

Kelly Clarkson sings far better than Christina Aguilera ever has.