I like that's it's AC/DC. Rock on!
That said, I feel like this weakens martial characters without adding much in the way of interesting choices.
Best case scenario, the martial is capable of both attacks. They either intuit the best one to attack or try both to determine the best. After that, they just stick with the best one. You basically end up with a golf bag fighter.
Worst case, they only have access to one of the two and are therefore less effective against half the foes they face. Maybe they're more effective against the other half? However, you probably don't want to go too much more effective than normal because of characters based on the best case scenario (otherwise, those characters are simply have that much easier a time than the default). However, if it's not effective enough, most players will probably just ignore it and use whichever type they prefer.
In 4e, there were four defenses so it wasn't anywhere as easy to target the weak defense. Also, the targeted defense was determined by the power you used, as opposed to the weapon you wielded, so it wasn't quite so straightforward a choice even if you did know the low defense.