Let's see. Carth has a bad past and whines about it constantly. Atton has a bad past and doesn't whine about it, even when the Exile discovers it. Carth is no fun to talk to, and Atton is hilarious. Carth's voice annoys me...okay, so does Atton's.
Carth professes his love to the PC while standing thirty feet away from her, and he doesn't even bother to kiss her despite the fact she's about to go to her death (probably)
. Atton's romance doesn't really have a satisfying ending, but that, IMO, is better than the end of the Carth romance. Carth is terrible in a fight, while Atton is great to have around.
Also, Atton is kinda cute, while Carth is...not.
Chapter 12 of A Soul Adrift
: On the planet Felucia, a young apprentice of the Dark Side thinks back to the beginning of her training as she lies in wait for her prey...
All the Time
: After four years in the Unknown Regions, the Exile returns to the known galaxy to visit an old enemy.
: A master of the Dark Side finds himself about to lose the one thing he cares about--and he will do anything to stop her from endangering herself.