Harden. I'd love to have Ibaka, but I think we're talking about good starter (Ibaka) versus star player (Harden). I know conventional wisdom says go big, but I don't think it's a tiebreaker situation.
Ibaka is an elite shotblocker and a good rebounder. Lousy offensive player.
Harden on the other hand is a few touches away from being a 20-5-5 player with tremendous efficiency (65% TS). He's got size, youth, athleticism, shooting, gets to the line, plays good defense.
I'd go after Harden with Pierce money after next year. Rondo / Bradley / Harden would be an incredible young core of "smalls". Draft for size.