I made it a defense feat of the player's choice because players may want to go for the Superior Fortitude/Will/Reflex line instead of the Improved Defenses feat. Likewise, there are now specific Expertise feats (e.g. Light Blade Expertise, Orb Expertise) that offer secondary benefits.
That makes sense. My concerns were for 'fixing the math' that those feats claim to do, and thus focusing on making sure that the math doesn't change with weapon choice, and that no defenses don't fall behind. You know your group, though, and the offer of a selection is probably a better idea.