oh man, this is a tough question. I love shawn Marion and think he is being underrated around here, and think these ridiculuos trade proposal of Marion for generic mediocre big men need to stop. Marion is perfect fit for this team as currently constructed. He will get his on garbage plays and fast breaks while allowing the set offense to concetrate on Amare, JJ, and Nash. Shawn Marion is a very, very productive player.
As for Artest, he is another guy I love. I love his toughness and defense. He has IT. He has that killer instinct that only Amare has on this team. I beleive that Artest is a superstar role player a la Dennis Rodman on those great Bulls teams. It is hard argue that Artest is stable. He is incredibly volatile.
In the end I choose the stability, athleticism, and continuity (for the Suns) of Shawn Marion over intangibles of Ron Artest.