I hadn't thought about the AC hit when raged. That would be an unfortunate loss on an AC-based prestige.
I was actually more interested in these questions from the DoS perspective, not a Kensai. For a paladin I'm comparing +6 str (really only 4 since you lose rage) and a boatload (where a boatload = about 200) of HP to the bonus hit/damage against evil outsiders. Seems to be coming pretty strongly in favor of Defender in most epics and the new raids. Though would obviously be closer in quests that actually HAVE evil outsiders... probably still Defender.
EDIT: And if the character happens to have less than 18 levels of paladin, it seems like a no-brainer to go Defender all the way.