Whose betrayal was worse for A) their respective vessels; B) the Federation (the possible ramifications for the future)? A) Ro Laren B) Seska Who'd win in a battle between the two on Deep Space Nine's Promenade? Each is equipped with a club. Odo, Q, and 9 Borg Drones are observing and placing their bets with Quark.
Seska was an entirely forgettable character. As evidenced by the fact that I've entirely forgotten about him (Her? It? Them?). I remember Ro Laren. That makes her better in every way. Including betrayals.
Seska hardly betrayed the Federation. She was a Cardassian operative who was undercover to infiltrate the Maquis, before being forced to serve on a Starfleet ship by circumstance. Ro was the bigger Federation traitor, though it definately was far more personal for Picard and the gang.
In regards to their respective vessels, Seska's betrayal was obviously worse given that she defected to the enemy and helped said enemy take over the ship. However, I don't think either betrayal had any serious effect on the Federation in anyway.
It's hard to say, as we know Seska's defection gave the Kazon an advantage, but it's unclear how much Ro's defection helped the Maquis.
Not necessarily...didn't Seska say something about how it would be to Voyager's advantage to form ties with the Kazon and all? While it's not explicitly brought up, I could see her thinking that if she couldn't get Voyager on good terms with the Kazon, she might at least be able to establish some positive Cardassian ties with the DQ if and when the AQ powers made contact.