I'd say it's super likely that custom character will be "lesser" experience as you put it. If you want to have the ability to fully roleplay as your custom characters, then a blank slate with no background gives you the most freedom to do so. I think Adams answers also point towards this direction. World will react to your class and your actions, but not any differently than they would to Origin character with same class and actions.
So if your class / race combo is available as Origin character, that's going to mean more content available for you than Custom character. However, then you are bound by that Origin story too. If your class / race combo isn't an origin story, then you will have an unique story that no Origin chraracter could replicate exactly.
I very much doubt there will be content locked away from Origin characters that can only be accessed by ALL custom characters. Since some classes/races aren't represented by an Origin character (probably?), those classes/races might give you some unique content only Custom characters can see.
