Hi, Sundance. Firstly - what do you want to use the revolver for??? If it's just to make a noise and have fun, either round is fine. If it's for defense against dangerous game, another set of criteria comes into play; if it's for general purpose handgun hunting, yet another; and so on.
If the weapon is primarily intended for defense against dangerous game, I'd strongly recommend the .475 as the way to go. It fires heavier bullets, giving deeper penetration than the .454, and has a greater frontal area. (I'm sure you know that mass has a greater effect on penetration than does velocity, and in solid bullets - which is what one would carry for preference against dangerous game in a handgun - the greater frontal area does more damage.) For more information on this, see the penetration tests from the latest Linebaugh seminar:
http://www.sixgunner.com/linebaugh/penetration_test.htm
If the weapon is for general purpose handgun hunting, the .454 makes more sense. It's powerful enough for defense against large animals, if needed; there is a wider range of bullet weights and styles available; .45 Colt rounds can be fired for fun, practice or self-defense applications; etc.
In barrel length, again, the application decides the length. For defensive purposes, a shorter, easier-packing barrel is more useful. For hunting, a longer barrel is more useful, given its longer sight radius, greater velocity to aid expansion of soft- and hollow-point ammunition, and easier fitting of a scope (if desired).
A quick question for those reading this thread. One can fire .44 Special loads in a .44 Magnum sixgun, and .45 Colt in a .454. Does the same apply to the .480 Ruger and the .475 Linebaugh? After all, the .480 is basically a ".475 Short". Does this mean that the .480 rounds could be fired in a .475 revolver? This would make the .475 revolvers much more flexible.
Facing the same big-bore issues, I decided not to worry about more powerful rounds, but to stick with my Redhawk in .44 Magnum. I carry the Garrett Hammerhead loads, which provide outstanding penetration and "killing power" for either hunting or defense, and have one heck of a track record (see
www.garrettcartridges.com). They are in the same power range as the heavier cartridges (on both ends of the gun!), but I don't have to buy a heavier and very expensive sixgun to load them. Just my $0.02 worth, you understand...
In closing, remember the immortal words of Clint Smith (of Thunder Ranch) on big-bore sixguns: "...He asked, ‘Did you hit him?’ Hey, I don’t know, but he was smokin’ when he ran outta here!"
Good luck and good shooting!