This seems like a good question though probably isn't. I would think the packing of powder would be an issue in a breech loader since a soft cartridge is loaded rather loosely into the breech, with no opportunity to ram home like with a muzzle loader. Doesn't a tightly packed load mean more efficient combustion?
I'm not really sure what your question is. By a soft cartridge do you mean a cartridge with a combustible linen or paper wrapping that contains the powder verses a muzzle loaded cartridge where one has the opportunity to seat the bullet hard up against the powder and, which system of loading would provide a better combustion and/or more velocity?
With the muzzleloader, you can pour more powder down the bore which should provide a higher velocity up to a point than you can with the breechloader which has a chamber which limits the amount of powder it can hold, (assuming you don't put so much powder down the bore of the muzzleloader to the point of either blowing up the gun or unburnt powder blowing out of the muzzle). So, the advantage of obtaining a higher velocity and longer range probably goes to the muzzleloader.
However, it's nice to hit what you're aiming at, and that would go to the breechloader. Its bullet was mechanically fitted into the bore of the rifle which allowed for better concentricity, meaning the bullet was better centered in the bore verses the undersized bullet of the muzzleloader.
From my experience with shooting black powder cartridge rifles, where you mechanically compress the powder charge in the case before seating a bullet, I do think you get a better, more complete combustion with compressed powder. But then, I was using magnum primers, so that may have helped a bit too.