Assuming that this hypothetical trade has nothing to do on whether they will draft Turner or Favors at #2 (the two most likely players for them). Who would have more value? Who would do better on another team? Who staying in Philadelphia helps the team? Who staying in Philadelphia hurts the team?
Elton Brand is a fairly mediocre player. He has lost his legs and is incapable of starting at the power forward position at this point in his career. He is a backup big man.
Andre Iguodala is an all-star quality player who effects games in multiple ways and can play 40 minutes a night.
There is a huge difference in each player's individual talent level and consequently a large difference in their trade values (Iggy has solid trade value, Brand is a large negative). Iggy offers far more to Phily or any other team than Brand does.
And, Brand is the guy Phily needs to move the most. Their big man rotation (Young/Dalembert + Brand/Speights) is clogged and Brand is getting in the way of their young player's growth + is hurting their defense tremendously and forcing them away from what made them successful in the past few years.