You might be right, Saric could turn into the next Illyasova: a bean pole of disappointing "potential." Or, he could bulk up a little, continue to improve his team D, and let Stevens put him in position to optimize his height advantage(s) and high motor. On O, I think Saric has a higher ceiling than Sully. I'm no expert, but from what I've read and seen, it seems like he would be a great fit with the C's motion offense, in terms of his passing, his work ethic, his shooting, and his handle. If Sully's ceiling is (as I think) a passable starter or solid role player, then it seems Saric's floor is relatively near Sully's ceiling. Saric's upside is in the All-Star range, and he does not have a history of back issues. I think his versatility and upside make him worth the risk of giving up Sully. In fact, due to Sully's back, keeping him might be even riskier than trading him for Saric.
McDermott seems to me like a Kyle Korver type. I'm not too excited about him. He flashed some unexpected athleticism at the combine/workouts, though from the tape I've seen, he doesn't use it in games. He gets blown by and dunked over on D (by college dudes, not NBA dudes). His O would be sick to have, though. I just wonder how much he could rely on his driving ability in the NBA, as unexplosive and unathletic as he moves. That said, I wouldn't mind him on the C's, just seems like Saric has a lot more potential, and again, his floor might be not too far from McDermott's ceiling.