It really depends how good Philly thinks Dunn is. If Philly thinks that Dunn is better than any PG in next year's draft class, then they're rather low on leverage. That team needs a PG. It also depends how ready Philly thinks Simmons is to run an NBA offense. He's got the skills to do so, but it's questionable whether he has the make-up. If they go that route, they really need to trade some bigs for shooters. A lineup with Simmons and two of Noel, Okafor, and Embiid has zero spacing. That could be some very ugly offense.
With the Celtics, is it better if they get a big instead of a guard? Yes. But if they draft Dunn, they can plug him into Evan Turner's role of running the offense of the second unit, and no more than half the cap cost. That lets them pursue major frontcourt free agents, and also makes players such as Terry Rozier expendable. Yes, the Celtics have a lot of guard-type players, but it's easier to play with a lot of them on the court than it is to play with that many bigs.
Unless the 76ers are not really impressed with Dunn after all, I think they have less leverage, because they need him more. Celtics can use Dunn just fine and shift different resources to finding a frontcourt players. The 76ers need to trade their bigs to someone for backcourt help, Celtics or otherwise.