It's pretty simple. New guy joins, or old guy creates an alias, he can jump right into a public world. If you want to join an established private world, he has to contact the commish, possibly answer a few questions and still may not get in.
As to which is better, it basically boils down to what you want. You have good private worlds that fill quickly, good private worlds that do not and poor private worlds that fall into quickly/not at all. In public worlds, you get what you get and have no say in who you compete against.