Since you have Mega Charizard X, I’m assuming this is Pre-Gen 8. Although it doesn’t really matter which in-game, I’d opt for White Kyurem.
•Your team has plenty of physical attackers already
•Black Kyurem doesn’t have Dragon Dance or any physical ice STABs outside of Freeze Shock, which isn’t the best move.
•White Kyurem actually has ice STAB in Ice Beam as well as superior dragon STABs in Dragon Pulse and Draco Meteor.
•White Kyurem’s access to Fusion Flare is valuable for beating steel types
•Kyurem has a superior special movepool to physical