Of the four jacket conversions I have done, only one—the one illustrated earlier in this thread—has antler buttons. The reason was simple, they go so well with that particular tweed and the plastic "leather" buttons that came on the jacket screamed "imitation" across the room. I rejected the faux stag buttons for the same reason noted by Tartan Tess—they look like plastic Oreo cookies to me.

I replaced the plastic suit buttons on two other jackets with with real horn buttons. Again, because the original buttons just looked shiny and cheap and I lucked into horn replacements at an extremely reasonable price. Real horn buttons have a low sheen that plastic has a hard time duplicating and you have that slight variation between each button.

I believe I have one conversion left in me, and the candidate has beautiful leather buttons I wouldn't think of changing. It's much more a matter of aesthetics than trying to copy a particular look.