The Omega requires the hammer to be manually cocked to full just before firing it, while the Black Diamond simply has a safety to slide forward to fire.
There's also an inexpensive weather shroud/breech cover for the Black Diamond:
http://www.shopwiki.com/Thompson+Cen...+Cover+%237337
One complaint about the Black Diamond is that finding a wood or laminated stock for one is extremely difficult.
I don't doubt that the Omega has the more modern design and is probably a better shooting rifle that's also available in more stock configurations. And if I were you I would probably buy one.
But since I like the option of being able to use either a #11 percussion or musket cap, I would still choose to buy the Black Diamond.
