Well, now that the gratuitous Searcy advertisement has finally come to an end, we can re-examine the original question: which of these two calibres is preferable?

With the .470, you will be locked in to paying premium prices for projectiles, and will probably have only the one bullet weight available. The .450 however can be fed a plethora of cheap .458 bullets from the trade, for endless experimentation with different weights, styles, etc, and cheap practice at the range. Many posters here will tell you that only the one bullet-weight will regulate in a double anyway, but that is usually untrue, I suspect especially for modern American-made doubles.

I prefer the .470 myself, its THE African big-bore cartridge for double rifles, but the .450 was the first nitro-express and is probably still the most practical choice today.

...that is, unless you are very well-off and can afford to buy Federal .470 factory loads, in which case why not buy both rifles!?!