I'm unsure of which to choose, and I'm going to say the locations and pros and cons of them:
Goodra: Route 14
Pros:
- One of the best pseudo legendary movepools
- Pretty good defenses
- Godlike Special Defense
- Pretty nice Abilities
Cons:
- Not pog in attack stats and Speed
- Pure Dragon isn’t that good of a typing
Garchomp: Route 13
Pros:
- Can be obtained right after Clemont and is going to be very useful because of his Ground type
- Pretty good physically
- Decent bulk and Speed
Cons :
- Not good with special stats in general
- Ice says hello
- Not a good Mega to choose
If you can, please suggest a moveset :)