Striker is banned because he is vitually 8/6 SOA
he is really strong and along with many cards he can do 2 hit KO even with 4 damage card (with fury)
and his SOA can penetrate many damage reducer
marina is strong, but her 5 damage is not really helpful to do 2 hit KO
That's like saying "who is better against Kolos, Marina or Striker?"
Both deal with the threat, in different ways.
Marina's higher power means that if Kolos threatens to KO with or without fury because you've already been weakened, you have a higher chance at winning. Marina also deals with Kolos being a damage reducer and can win a no pill or low pill fight as well.
Striker's SOA takes Kolos' damage down to 4, which makes Kolos hardly a threat, and can threaten 6 damage making it worth it to pill slightly. However, he can't expect to win with 1 pill and if you're 4 damage off from a KO, Kolos is still a threat.
Similarly, they work in different ways to deal with all the other cards in the game, and each has their own strengths and weaknesses, and both can be played offensively and defensively.
If you're playing mono use both definitely. If you're playing a half deck you have to choose based off of the other clan.
@Tanto - Striker is not "virtually an 8/6". Look at Caelus, for example: when he reduces a card with 7 Power to 2 Power, he has twice as much - in reverse, he would have to have 14 Power to produce the same effect. There's a huge difference between +Power and -Opp. Power abilities.
Anyway, Striker's the better card - he has more 2HKO opportunities, and his SOA makes him more reliable than Marina.