As I always say, it depends on the deck. If you're playing Mono All-Stars you have no excuse not to use both (and throw in Asporov while you're at it). If you're running a half deck filled with 6 damages (looking at Fang Pi Clang, La Junta and Uppers), run Striker. If you need an SOA run Striker. If you need a reducer run Marina. If you're running a low damage half deck such as Junkz (generally, especially since Junkz has 3 viable DR and less than stellar SOA) Striker may be a very good idea. Running Pussycats? Striker. SOA clan? (Roots/GHEIST), Marina may be for you (though debatable, since Striker 2HKOs with Shakra, Anibal, Rolph, and several 4 damage cards with Fury).
It's completely situational, and sometimes you might find it better to cut corners in the other clan in a halfdeck to fit both in.
Marina's the best. Whenever I face her in ANY deck she poses a great threat... 1 power > 1 damage in terms of raw stats (When I play my GHEIST deck) and -3 opp. damage min. 1 > SoA IMO. Maybe not, maybe cause I play GHEIST alot and is used to the SoA always being there...
I first study my other half.
if the other half lacks DEFENSE, definitely Marina ftw.
if it lacks OFFENSE, definitely Striker.
you can't really say which is better.
it all depends on how you play, and what is really needed.
otherwise, 7 power for an All Star is huge, so my vote goes for Marina.