Archeops can contribute to your team by learning Fly and Cut. Also it can be a major powerhouse in your team with that amazing Attack and Speed and great movepool. Archeops can use Physical and Special Attacking moves which is pretty amazing. Archeops has a flaw, which makes it terribly weak which is its ability Defeatist which Halves Attack and Sp Attack when its HP is below half. That will be a problem considering that Archeops has very frail Defenses.
Carracosta can contribute to your team by learning Surf, Strength, Waterfall, and Dive. It can also be a Powerhouse in your team with that Amazing Attack and Defense. But Carracosta can only run Physical moves because it has higher Attack. Carracosta only has one flaw being that it has a 4X weakness to Grass types, but not a major issue. It has the Ability Solid Rock to weaken the power of Grass type moves.
In my opinion, you should go with Carracosta because it can learn more HMs and it can counter the flaws that it has, unlike Archeops. Hope this helps.