The Brave Blade starts at 150; if you never run from battle, it remains at that number. It cannot go higher in a normal game. The Chicken Knife maxes at only 127, but because of how it calculates damage is in almost all cases actually more damaging than the Brave Blade (it takes Agility into account). It also forces one to flee battle ~25% of the time.

As I suggested earlier, it's a rather simple decision. If you never escape, go for the Brave Blade. If you frequently retreat from battle, take the Chicken Knife.