Other: Both, if we're going to keep both as stats.
4e did its non-AC defenses much better than 3e or 5e. There aren't too many of them (just 3), they draw on multiple stats (so it's rare for a character to be bad at too many of them), and they were difficulty values hit by attacks, rather than bonuses to rolled saving throws. These are all significant improvements, and obviate any need to worry over whether "willpower" comes from Wisdom or Charisma.
If I had my druthers, though, I would reshuffle the stats themselves. Strength and Constitution would merge together as Might. Dexterity would be mostly left alone, but decoupled from Initiative, which would just be a separate stat (perhaps Agility or ). Intelligence would absorb any remaining academic characteristics of Wisdom, while Charisma would absorb the "willpower" angle. That would leave Wisdom reduced to just Perception, which is perfectly acceptable given how powerful Perception is as a characteristic.
Might, Dexterity, Agility, Perception, Intelligence, Charisma. Perhaps swap the second and third positions there, and it can be abbreviated as MADPIC. Might breaks free of restraints and powers on despite fatigue. Dexterity is precision, grace, fluidity of motion. Agility is speed and dodging. Perception is your ability to detect what is real and reveal what isn't. Intelligence is everything about academics and reasoning. And Charisma is willpower, sense of self, and manipulation (friendly or unfriendly).
Alternatively, if one is willing to abandon the six-stat model, make Initiative and Perception completely decoupled from ability scores, and reduce it to Might, Dexterity, Intelligence, and Charisma.