Kyurem-W is better. Reason?
It can abuse its huge 170 SAtk with STAB: Draco Meteor, Dragon Pulse, and Ice Beam. Kyurem-B has det sexy Outrage, which can rip through walls, but it is forced to go mixed due to it not having a reliable Physical Ice STAB.
Kyurem-W's combination of Dragon Pulse / Draco Meteor and Fusion Flare hit unresisted throughout the tier, due to Turboblaze.