Thanks for all the info guys. As far as the 24" vs the 20", I assumed the 24" would give me better accuracy and a little longer effective range.
All other things equal, shorter barrels give better accuracy because they are stiffer. Read up on barrel harmonics to understand.
As to muzzle velocity, the 45LC is a pistol cartridge and is typically loaded from the factory with fast powders. This being the case, it is logical to assume that all of the powder will be consumed before the 20 inch mark is reached and that by 24 inches, the bullet has already begun slowing down. This phenomenon can probably be mitigated with a different powder selection when reloading.
Speaking of reloading, if you are not already a reloader, you will probably want to become one when you start shooting 45LC. It is without a doubt the most expensive handgun ammunition extant in my area. At $40/box of 50, I couldn't afford to shoot it very much.