Kyurem-W is better. Why, you may ask?
It can abuse its huge 170 Special Attack with STAB: Draco Meteor, Dragon Pulse, and Ice Beam. Kyurem-Black has that awesome Outrage, which can rip through walls, but it is forced to go mixed due to it not having a reliable Physical Ice STAB.
Kyurem-White's combination of Dragon Pulse / Draco Meteor and Fusion Flare hit unresisted in the tier, due to Turboblaze.
(Note that this is for competitive, in Ubers.)