They're not equal players. Iggy is a 2/3. Deng is a 3/4. Deng is 6-9. 220. He's taller than Brand, but is outweighed by Elton. Iggy is only 6-6 and 207.
The Sixers will get a huge boost out of Iggy this year if they can keep him. For most of his career he's been forced to play the 3 spot because the Sixers were short on their front line.
It was a terrible mistake by Cheeks to keep Iggy at the 3 against the Pistons. Iggy had real problems getting by Tayshaun Prince. Against Billups or Hamilton he would have been too much for them to handle. The Sixers had the opportunity since they now have Thad Young, who is a more natural 3. Had Cheeks done that, as he did during the regular season when they beat the Pistons, the Sixers would have had a much better chance at an upset.
The addition of Brand adds depth to the Sixers front line and allows Andre to play his more appropriate position. Having a strong option at the 3 like Thad Young now forces Cheeks to play him at the proper spot (where he should have an edge) Iggy is good enough to be a 3, but can be physically overmatched, giving up a couple of inches and a couple of pounds to his opponents. Against 2's he'll be dominant most nights. Against Boston that's the difference between being guarded by Paul Pierce or Ray Allen. Pierce is bigger and stronger than Iggy, but Iggy is bigger and stronger than Ray Allen, and it makes all the difference between winning and loosing.
The best teams are the ones who have better players at more positions and more depth. The Sixers if they are able to keep Igoudala and Williams have made a quantum leap up. Just how far up depends on how well they play together and adjust to Brand. The nice thing is that Brand is surrounded by players who are mostly younger than he is, and they should only get better with experience. Their development should be hastened by having a team-mate as dominant at his position as Brand.
If we didn't have Thad Young, the Sixers might well have made a run at Deng, but seemed to prefer Josh Smith.