If you reload, the 44-40 sucks, and the brass is a lot harder to get, also. Not to mention finding good Factory Hunting Ammo. I'd go with the 45 Colt, any old day and twice on Sunday!
^^^^^^^^ this
I'll agree that it is far simpler to run the 45 Colt brass through a carbide die, but the .44-40 isn't that hard to reload for. Yes, case mouths are thinner, so just start your brass straight instead of cockeyed. And, get a Lee factory crimp die and you are golden.
With low pressure loads the .44-40 seals much better due to that thin neck, keeps the action a bit more clean. The .45 Colt won't seal.
I have a '73 in .44-40 and a '92 in .45 Colt. Like them both. But in a '66 I think I'd opt for the .44-40 I deference to the action. Now, if the OP was getting a '92 I'd suggest the .45 Colt as it is more versatile in that stronger action.
