I think Orien got drafted because he's got better overall NBA potential. Tiras is a great scorer, but he's a terrible ball handler and he lacks on the defensive side. He's great from the charity stripe and is a fairly consistent shooter, but when you pick him that's what you're getting and that's all he'd end up ever being. He would have been a GREAT fit for a team like the Mavs or Suns, though, where the focus isn't necessarily on defense but on scoring. I really hope he ends up on one of those teams and proves me wrong and becomes an all-star.
Orien Greene, on the other hand, is a great defensive player, he can distribute the ball, has a good mid-range shot, can shoot the 3, and penetrates very well. He's generally a very unselfish player and that's very important. He's can be a bit streaky shooting, but under the right coaching - which I think Doc Rivers is - he can be molded into a solid NBA player.