Let's compare the two and I'm only gonna put the significant differences between themm
Flygon
- has a switch move in uturn
- has access to dragon dance
- an incredible ability in levitate
- has access to defog
Garchomp
- all of its non spa stats are higher
- has access to stealth rock
- access to swords dance
- can perform a defensive role with rough skin rocky helmet chip damage
Those are pretty much the significant differences between the two
If you're playing competitive, the answer is clear. Garchomp is the better of the two. Chomp is ou while Flygon is only ru in the smogon rankings. In doubles, both of them have it rougher as they have to compete with the omnipresent Landorus Therian but without Lando, Garchomp is still the preferred ground type. Here is a usage stats from vgc 2017
https://www.nimbasacitypost.com/2017/08/teams-and-results-from-2017-world.html
In game, the scales are further skewed in Garchomp's favor as the land shark just makes any Pokemon game easy with its above average bulk, attack and troll speed. Garchomp is trickier to get since you would have to sos a Gabite but by in large, it is far better than Flygon. In game, attacking is far more important than utility or defense so Garchomp's higher attack and speed gives it the edge
At any rate, here are some sample sets for them
Flygon @ Choice Scarf
Ability: Levitate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Uturn
- Dragon Claw
- Earthquake
- Stone Edge
Garchomp @ Rockium Z / Dragonium Z / Groundium Z
Ability: Rough Skin
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Claw / Stealth Rock
- Stone Edge / Outrage
- Earthquake
- Swords Dance
Just to clarify the movesets, ignore the items and evs if playing in game. On Garchomp, dragon claw is strictly for in game only. Do not use this move for singles competitive and it is even debatable if it is needed in doubles. Stone edge is slashed with outrage as you can choose which move you want to use as your nuke
