I am a big Ibaka fan, but I think Collison will do a better job on Dirk. Ibaka has a tremendous defensive skills and is a freak but sometimes makes mistakes. Especially when he try to block shots and allows easy passes. Collison is more experienced and is a very smart defender and currently can do better in difficult situations. And defend Dirk in a conference finals is perhaps too much responsibility. Still does not mean that Ibaka can not be one of the best defensive players in the league, in fact it already is.
Defense on him consists largely of ball denial and not letting him establish position. Once Dirk has you on an island all you can do is stay on your feet and be physical. Even putting a hand up is borderline futile.
Dirk I think would have had a tougher time with the Grizzles. All those small quick defenders upsetting his rhythm and stealing the ball. And more importantly, Ibaka won't make Dirk work defensively to the extent that Randolph would have. Neither Ibaka nor Perkins are really big scoring threats, leaving lots of energy for Dirk and Chandler.