Since this is in-game, it doesn't really matter.
For in-game, just pick what you want to use more, maybe Haxorus since you can get better moves in it without breeding. But I am going to answer this for competitive as well.
Salamence for competitive.
My reasons being:
- Higher BST(base stat total)
- Higher special attack(if you want a mixed or even special set)
- Higher speed
- Moxie as an ability
Two sets I used in the past:
Salamence @ Choice Scarf
Ability: Moxie
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Outrage
- Dragon Claw
- Earthquake
- Fire Blast
Salamence @ Life Orb
Ability: Moxie
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Dragon Dance
- Dragon Claw
- Earthquake
- Fire Blast
And a mixed set in case you were interested in it:
Salamence @ Life Orb
Ability: Moxie
EVs: 64 Atk / 192 SpA / 252 Spe
Naive Nature
- Draco Meteor
- Outrage
- Fire Blast
- Earthquake
Source: experience using Salamence a lot in Gen 5
Hope I helped.