Archeops can cover fire type which you don’t have coverage for. On top of that, it covers for Fighting types with its flying type moves (which Carracosta lacks). They both have mediocre movepools. Unlike Archeops, Carracosta can learn Ice type moves to cover for Dragon types. This means Carracosta is useful for the Opelucid Gym. But are Archeops is useful against Alder’s Bug type Pokémon, though Archeops has a pathetic ability (Defeatist), and Carracosta has a good one (Solid Rock).
Now, it may seem like there equal, practically balanced BUT Archeops has one upper hand. Its stats are… BETTER than Carracosta. Archeops has a total base stat of 567, whereas Carracosta only has a lowly 495 :( . This stat superiority puts Archeops on top, it has great Speed with 110 and has great Attack and SpAttack with 140 and 112 respectively.
Overall, Archeops is your best option.