Zoroark
- Found earlier
- Gets boosted experience
- Evolves much earlier
- Learns moves like U-turn, Flamethrower, Dark Pulse, and Grass Knot
- Faster than Haxorus (and a lot of other Pokemon)
- Massive Special Attack
- Illusion really screws with Caitlin if you play it right
- Very, very frail
- It can disobey you if you level it up too much
Haxorus
- Axew is found just a little later than Zorua
- Absolute monstrosity, with 147 base Attack
- Decently fast
- Learns moves like Dragon Dance, Dragon Claw, Strength, Poison Jab and Aqua Tail
- Really helps to slaughter more of the late game (Ghetsis, Drayden, Iris, etc.)
- With Mold Breaker, it laughs at the plethora of Sturdy Roggenrolas/Boldores
- Evolves much later
- Has to evolve twice to become Haxorus
- Not altogether useful at first
I'd go with Haxorus. It's a beast, and it helps a lot more in the long run than Zoroark does. Here's a set:
Haxorus @ Life Orb / Muscle Band
Ability: Mold Breaker
- Dragon Dance
- Dragon Claw
- X-Scissor
- Rock Slide / Return
Hope I helped!