Neither. Fusion Bolt outclasses both in this situation. But if you want one of those two, it varies. If reliability is what you want, Thunderbolt. If power and side effect with 95 accuracy is what you want, Thunder Fang. Ingame, Zekrom can take a hit easily if he misses so Thunder Fang is better if you think about Zekrom's future rise in his Attack Stat.
If you want it between those two, Thunderbolt. It's reliability in accuracy is greater than Thunder Fang, and the difference in BP is noticable. However, by the looks of it, your Zekrom is not level 100 yet, so in future levels if you don't EV train Speacial Attack, it's Attack will rise higher than it's Special, making Thunder Fang more viable. However, if you have access to Fusion Bolt, it is much more effective than the other two.
- If you can get Zekrom to learn Fusion Bolt, this is the best option. If PP is really a fuss, PP Up it or use another move.
- Thunderbolt for reliability, but weaker power.
- Thunder Fang for less reliability (not by too much though), but higher power on average. And the 10% chance to flinch as well as paralyse with another 10% is always a help.