To the OP:

You are going to make a heck of a mistake if you buy a .470NE or .500NE for your first double rifle.

Only if you are used to shooting some real stompers in your bolt guns will you want to tackle such a large caliber.

If you intend to buy a double rifle for shooting you should consider a medium bore rifle. An excellent choice would be a 9.3x74R in a nice double. It's plenty for any North American game and won't beat you up like the calibers you are interested in.

Curl