Yeah, the Finish
--
-The finish on the more expensive weapon, as Hawg said, is the difference.
If you are shooting CAS, you'll need two, which translates to $400 more.
CAS guns get used a lot: handling, shooting, cleaning and transporting.
The bone charcoal finish is in my opinion the absolutely most beautiful gun blue finish obtainable. However, it is also pretty fragile compared to other finishes. As it wears, the charcoal blue will weather into that old silvery look you see on old examples of Colt SAAs. Case finishes will also weather out over time.
However, the bone charcoal blue is also susceptable to damage from everyday items we sometimes use to clean with or handle near the guns, such as vinegar and stuff containing vinegar, Coke and other soft drinks and solvents containing various chemical elements. It also will accumulate holster wear faster than other finishes. Keeping a nice coat of anhydrous lanolin on the guns can help considerably, but is a stopgap measure.
I'm not saying you should not get the charcoal blue finish, but you should be aware of what you are dealing with.