Muzzleloading is different from shooting modern cartridge guns. It is really a combination of reloading and shooting and there is much to learn.
To my way of thinking, a person just starting out n this game should try to keep things as simple as they can at first. They need to learn about ball size, patch thickness, lubricants, powder types and their uses, how to clear a mis-firing load, how to start and ram a patched ball, proper cleaning methods and a number of other things.
A large number of people live in areas where real black powder is not available and they do not want to buy 5+ pounds of black powder.
For these people, IMO the Percussion gun is the best choice for starting to learn all of the things they will need to know.
A Percussion gun is fairly easy to shoot and if it is loaded correctly is quite reliable.
After they have learned to use their Percussion guns they can move on to the Flintlocks and continue their learning curve including flint knapping, flint positioning, flint selection, pan priming, vent picking and ignoring a flash of powder 12 inches from their nose before the gun actually fires.