I think both characters strike an interesting parallel. Carth Onasi is a decorated Republic war hero, and Atton Rand... well...
he used to be a Jedi killer.
Both Carth and Atton are some the first party members in KOTOR and TSL, respectively, so they both stand as veterans, in terms of being your character's companion. Both have pasts they'd rather not speak of, adding elements of mystery to their character - this compelled me to converse with them, in order to discover the truth.
Carth is a soldier, and Atton is a scoundrel, and though they both use blasters, they have different talents.
Which one is better?
Neither; they are both unique and excellent characters - I couldn't choose one over the other.