In a straight comparison, I would expect the defensive fighter to handily defeat the offensive fighter, because defensive fighters are pretty much only good against other fighters while offensive fighters are equally good against everyone. To complete the triangle, shield should beat greatsword because greatsword beats staff and staff beats shield.
This is an interesting point. It slightly assumes defenses are against weapons (AC, resistance vs. B/P/S, etc.) and not generic (resistance vs. all, save bonus, temp HPs), but that makes sense from what you can do martially.
So facing monsters, if a defense will be applicable only about 1/2 as often as an offense, it should be twice as powerful. This will balance them in the normal GvE play of the game. It will mean that all else being equal, a fighter with a defensive fighting style, since it will likely apply against what another martial character is doing, should handily defeat an offensive based fighter. Anything else, combined with the less general applicability of the defense, would be significantly underpowered in general play.
Mind you, the same is true though to a lesser extend with offense. For example, melee is not as generally applicable as ranged (some actions lost moving, not against those with some other movement capabilities like flying, loss of offense if backing off due to badly hurt), so melee should do more than ranged.
Tacking on my earlier teamwork based features edge, and it seems that there are clear guidelines how to balance these out. It's not just power, but also how applicable they are in Group vs. Environment play. PvP really has nothing to do with it.
