The lens difference makes sense. The difference in the frown still seems odd. The idea that the stunt needed a larger frown because of the distance may be an explanation, but that certainly doesn't fit with how they used the stunt helmets.
Other than the lenses, I can't think of anything that actually makes the stunt version better for stunts. Things like weapons are way more obvious. The stunt version of weapons frequently look really bad up close. Ironically, someone would probably be rejected for 501st approval if they tried to use most screen used stunt items. They generally aren't meant to be seen up close.
The arguments about the number of rivets on the holster seem a little odd. I understand the ones with two rivets generally went with stunt armor and the ones with four rivets generally went with hero armor, but it appears they didn't always keep them as complete distinct sets of armor during filming. The film makers didn't seem to care what combination appeared on screen, but people trying to make screen accurate armor do care.