Garchomp
- Very high Attack
- Decently bulky with 108 HP
- 102 Speed is pretty fast
- Learns moves like Dragon Claw, Poison Jab, Stone Edge, Crunch, Earthquake, and Aqua Tail
- Special Defense is somewhat lacking
- Crippling weakness to Ice moves
- Found mid-game and evolves late
Hippowdon
- 108/118/72 defenses give it incredible physical bulk
- No 4x weaknesses
- Attack stat isn't bad
- Learns moves like Fire Fang, Ice Fang, Thunder Fang, Crunch, Earthquake, Stone Edge, and Iron Head
- Very slow
- Fewer options than Garchomp
- Special Defense is not good
You should go with Garchomp. It's more offensive, which matters much more in-game. Here's a set:
Garchomp
Ability: Rough Skin / Sand Veil
- Earthquake
- Dragon Claw
- Poison Jab / Aqua Tail / Brick Break
- Crunch
The other fully-evolved Ground types are Diggersby, Wormadam, Krookodile, Rhyperior, Steelix, Marowak, Golurk, the Nidos, Dugtrio, Flygon, Golem, Quagsire, Stunfisk, Whiscash, Mamoswine, Sandslash, Gliscor, and Zygarde.
Wormadam, Steelix, Flygon, Quagsire, Stunfisk, Sandslash, Gliscor, and Zygarde are all bad for one reason or another. Of the ones left, the best choices for a Ground type are Diggersby, Krookodile, and maybe Dugtrio. However, if you're willing to go a long time without a Ground type, then Gabite, Dugtrio, and Krokorok can all be found within the same region. Garchomp is better than both of the others, so you should still use that.
Hope I helped!