Pros and cons.... If you use the .45 Colt brass, you'll leave a slight ridge in the chamber at the mouth of the case. It's not that big a big deal, though; it's pwder and lead residue that can be cleaned off with a little scrubbing. The more you shoot and the longer you let it sit, the harder it is to clean off. If it is allowed to build up and not cleaned, you can't chamber a .454 - the case (longer than a .45 Colt) stops when it hits the ridge.
Light (.45 Colt) loads in .454 brass leave you with a whole bunch of unused space in the case. This can cause very uneven ignition with the accompanying velocity variation, and even the occasional misfire. Set up a chrono and fire some of these loads by first tipping the muzzle up (putting all the powder right next to the primer), then gently leveling the gun and firing. Then tip the muzzle down, gently level the gun, and fire. Different primer/powder combinations will exhibit varying degrees of sensitivity to this; some don't seem to mind too much, and some show wild changes.
The same problems occur with .44 mag/.44 Spl and .357 mag/.38Spl