I'll give a Pros and Cons for both and you can choose
Honestly neither one is "better" than the other. I will compare them and let you choose. (plus I'll add in movesets :D)
Metagross
It has better stats (600 to 530)
It has a better ability.
It has a better movepool.
It has better typing.
But all of that is before the mega...after the mega some things change
Aggron gains a better ability
Aggron gains better typing.
Here are my suggested movesets for both, before and after the mega.
Metagross @ Life Orb
Ability: Clear Body
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Agility
- Meteor Mash
- Earthquake
- Zen Headbutt
Metagross @ Metagrossite
EVs: 252 Atk / 4 SpD / 252 Spe
Hasty Nature
- Meteor Mash
- Earthquake
- Zen Headbutt
- Bullet Punch
Aggron @ Aggronite
Ability: Sturdy
EVs: 252 HP / 16 Def / 240 SpD
Impish Nature
- Rest
- Sleep Talk
- Heavy Slam
- Earthquake
Aggron @ Chople Berry
Ability: Rock Head
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Autotomize
- Head Smash
- Aqua Tail
- Low Kick
Hope this helps!