My question would be why would the OP want details on the difference between the .470 NE and the .450 NE.
Both rounds do essentially the same thing.
In today's world, the .470 is much more common, both in terms of loaded ammunition and in terms of loading components. However, the .450 is not hard to provide for either.
Obviously, if you are considering the .450 you are approaching it more as a collector than as a hunter. If you acquire it you will have two dangerous game double rifles, either of which would serve you well. I am of that vein. I have numerous dangerous game rifles.
So if you are a collector, buy the rifle if it appeals to you. The .450 NE is a great round. If you would like to see mine, have a look here:
http://rbsiii.com/collection/rifles/gibbs_450/Curl