I lean towards origin character narratives currently because from what we get in the EA is seems clear that playing a custom character, or a squad of Tavs, feels like choosing to paint without the color red, the origin characters are going to be more realized people, they'll have motivations and history. The argument for a complete blank slate to roleplay onto never really sways me because unlike in a game with real intelligent rule masters, these games are only ever going to be so reactive to your decisions. This has been the direction crpgs have been going since Mass Effect(?), and I can't really complain too much about it, it can create moments of dissonance between your intention and your avatars actions but that comes more down to how well the game is creating scenarios for you to work in.
If more was put into the development of an MC's background to make them effective stand-ins to the Origin characters, then my opinion would change but that still doesn't seem like what the origin skeptics are really after.
I think a large part of what makes a video game RPG more interesting than linear narratives and even table top games is that ability to replay it, changing variables and seeing how well the world has been written to react to those changes. I'm looking forward to replaying BG:III as different origin characters and seeing what impact that can have on each distinct playthrough.