I've fired a .454 once (i.e., one shot), and never shot a .480, so obviously I'm no expert on this, but, for what it's worth, I'd take the .454 for the following reasons:
1) It is more versatile because you can shoot .45 Long Colt as well as full-powered .454 in the same pistol.
2) It is more powerful than the .480 when using full-powered loads (but I suspect you could load it to more or less match the .480, .44 Magnum, or any similar cartridge).
3) It is a better established cartridge (more likely to still be somewhat popular in 10 years).
I'll be interested in hearing what others think (I've been thinking of getting a .454 myself), and I'm very open to being corrected if any of my reasons are incorrect.
Doug