I would consider availability of ammo in your decision. Hornady now makes ammo for doubles and that is a BIG plus when purchasing a DR.
I'd choose a 450/400 NE 3" and a 450 NE 3 1/4"
Another plus for the .450 is availability of cheap .458 bullets which can be used to reload so the owner can shoot more, practice and enjoy his rifle.

With a fine gun on his arm, a man becomes a sporting gentleman, both on the field and off.