I've seen these listed as such, but I really thought they were the exact same. Am I right? IF they are different, how so?

No difference.

However there are different alloys, such as titanium, SS, ect...  One who owns a FA may opt for a SS for the strength while one with a varmint AR may want a titanium for faster lock time.

There is a difference in the size of the collars on the AR and M16 pin.M16 collars are larger almost to the same diameter as the inside of its tract in the carriewr.AR15 semi auto pins have a smaller collar and before shrouded bottom carriers or the use of M16 carriers when they were machined away exposing the rear of the pin a large collar pin would take a beating from being exposed and hit by the notched hammer.On an unshrouded carrier the rear of the pin and collare would make contact cocking the hammer instead of the carrier itself like on M16 and shrouded bottom carriers.With a smaller collar pin with open bottom carrier just the rear of the pin would contact the hammer.But generaly thats the big difference....pin colar diameter.

I have never tried a SS pin but Titanium is garbage and will eventualy break...its harder but more brittle.I have chromed pins that are still going after years and thousands of rounds.So either M16 or AR pins will work in a FA carrier but I wouldnt use an M16 large collar pin with and unshrouded carrier.