Well, it all depends on your style of play. Defensive or Offencive.
Tirtouga is an excelent defencive pokemon, with Solid Rock as the ability, it can be a 'tank' pokemon, absorbing most of the weaknesses. Evolve it into Carracosta, and you have a Buff-Tank pokemon.
Archen is a berzerker that needs to be guarded carefully because of its ability Defeatsist. Extremely high attack for the little guy's size, however. Evolve it into Archeops and it's a jet fighter, with a very high attack and very good speed.
So it's really up to you. What do you want? Or what does your current team lack? A tank, or a Jet fighter?