There are ergonomic issues to consider in your rifle choice besides the weak toggle link systems in these 2 firearms. The 1873 has a wood forearm for your hand to grasp while holding the rifle. With the 1866, you are holding onto the cartridge tube and barrel while firing. If you shoot more than a few rounds at a time, the barrel will get very warm, maybe even hot enough so that you can't hold onto it for awhile until the barrel cools down. So the 1873 would be my first choice of the two. But I agree with other posters that a 1892 would be the absolute best choice in a handgun caliber lever action carbine. It will last your lifetime and that of your grandchildren with minimal good care.
As to cartridge choice, there will be no to minimal ballistic advantage to a 38 special in a rifle length barrel compared to a handgun with a 6 inch barrel. You might even lose velocity in a carbine or rifle length barrel with the 38 special because the powder used in 38 special cartridges is very fast burning, and is not sufficient to overcome the friction of a rifle length barrel to gain velocity. In short,you will not gain any velocity with 38 special cartridge in a 1866 or 1873 carbine, compared to a handgun. A 9mm cartridge is about halfway in power between 38 special and 357 magnum with a 124gr. Bullet ( NATO or +P ). So a 38 special is not quite equal in power to a 9mm cartridge.
Based on the above information I would recommend the 1873 ( or better yet an 1892) replica carbine in .357 magnum. For fun and practice shooting I would use 38 special wadcutters. For home defense loads I would recommend premium 38 special +p or +p+ self defense loads, or .357 magnum loads. If they are available in your country I would recommend Hornady critical duty ammo ( 1st choice ), or Critical Defense ammo ( 2nd choice ) for your home protection ammo. Federal makes good general purpose ammo for practice shooting, and good .357 ammo for fun or self defense.
Good luck.
As to cartridge choice, there will be no to minimal ballistic advantage to a 38 special in a rifle length barrel compared to a handgun with a 6 inch barrel. You might even lose velocity in a carbine or rifle length barrel with the 38 special because the powder used in 38 special cartridges is very fast burning, and is not sufficient to overcome the friction of a rifle length barrel to gain velocity. In short,you will not gain any velocity with 38 special cartridge in a 1866 or 1873 carbine, compared to a handgun. A 9mm cartridge is about halfway in power between 38 special and 357 magnum with a 124gr. Bullet ( NATO or +P ). So a 38 special is not quite equal in power to a 9mm cartridge.
Based on the above information I would recommend the 1873 ( or better yet an 1892) replica carbine in .357 magnum. For fun and practice shooting I would use 38 special wadcutters. For home defense loads I would recommend premium 38 special +p or +p+ self defense loads, or .357 magnum loads. If they are available in your country I would recommend Hornady critical duty ammo ( 1st choice ), or Critical Defense ammo ( 2nd choice ) for your home protection ammo. Federal makes good general purpose ammo for practice shooting, and good .357 ammo for fun or self defense.
Good luck.