That is retarded. Bad teams have all-star players, superstar players all the team.
A team's success does not depend solely on the level of talent they have playing. There are a lot of intangibles in it, and the argument Boris Diaw is a bad player because he wasn't getting played on a bad team that didn't understand is game is silly. VERY silly indeed.
As to the question at hand, at this point I'd have to go with Diaw. He's younger, has a better contract, and is a MUCH better playmaker which the Suns will need when Nash's decline begins. His midrange jump shot is more consistent then Marion's, and while not a rebounding freak like Shawn, he pulls down enough for his position. He shoots about the same percentage as Marion, Marion has better range, and before this year Marion was a much better defender. But after seeing Lamar Odom, Sam Cassel, Dirk Nowitzki, and almost any elite scorer he was defending torch him during the regular season + playoffs, I think Marion is forgetting what got him that fat contract and relishing his 22 PPG scoring average.
But most importantly, Diaw averaged 13 for the year, Marion 22. Against Dallas, the Suns biggest test, Diaw goes for 24 a night, Marion 17. Diaw also nabs the game-winner in game 1 away from home.
Don't get me wrong, I love Marion, but a lot of what he does, play the PF, score a lot at a high %, weak side blocks, rebounding, will be largley negated by Amare, and to a lesser extent Kurt Thomas, coming back.
And none of this even mentions the fact that this is Diaw's first real season with serious playtime. He could get much better then he was in the regular season, he certainly showed flashes in the postseason. With Marion you pretty much know what you're gonna get. His game is heavily based on his athleticism so I wouldn't be surprised to watch his productivity dwindle as he gets older.
So, if I had to choose for the franchise, Diaw. Marion is a better player now, but I don't think you'll be able to say that in 2 years.