How about none of them! Lou, Marreese, and Thad are all potential stars and a good basis for a championship team. I'd rather be patient and let them develop rather than win now. I think with a little luck they, along with Andre Iguodala, will all continue to improve. Even if Thad and Igoudala play the same position, I don't see a problem. There's not that much difference between a 3 and a 2 anyway and with a 6'8" and a 6'6" player a height advantage against most teams.
If you have to trade anyone, trade Dalembert for another Center. Let me know when you find one that will give you 10 and 10 a night like Sammy, without being error prone. Maybe that's what Jason Smith will be, so you can get rid of Sam for an outside shooter next year.
I wouldn't be in a rush to get rid of Miller either, since DiLeo became coach his assists have gone way up. He's still a decent scorer, especially posting up defenders, something the Sixers need more of.
Lets see what happens with the Sixers over the next 2 weeks as they travel out West. See if they can play the kind of defense they need to win games, as well as score when they need to.