I'd trade Zeller because Covington fills a position of need, is a good-enough 3pt shooter, and has the tools to be a good defender. Philly would possibly be interested in a traditional center to spell Okafor, allowing Noel to play most of his minutes at SF.
I would not trade Turner, he is the far superior player, and will prove to be very valuable to this team when they can't score in the playoffs.