jeez, if only game director Todd Papy were here to explain how each god works, that'd be swell.
oh wait! Here he is! (specifically at 3:56)
Ares: main tank
Posiedon: balanced croud control support
I know he has all of the good things because, he has physical resistance, strong physical attack, and all that because it said it on a TON of post everywhere else. And Poseidon looks BOSS.