I have used both barrel lengths in both calibers extensively. I prefer the extra sight radius of the 6", and personally, prefer the .475 to the .454 for my own use. These are highly personal and subjective decisions, though, and there are those who will disagree, based on what and where they hunt and their experience level. Cases and components are available, although you may have to source them via the Internet from someone like Midway.
You didn't mention it, but I actually prefer the .500 Wyoming Express to the other two. Those cases however, are only available from FA.
Any of the 3 will kill whatever you want.