Interactive fiction may have its (popular) roots in choose-your-own adventure games, but the main appeal in CoG is being able to play as a character that you define. There are very few games outside of a very niche market that have playable characters outside of the “norm,” such as nonbinary, LGBT+, or non-Caucasian characters. This, I think, is what draws many new players in, almost as much as the premise of Choicescript and narratives being defined by player choice.
While it is true that many IF works have the so-called “blank slate” protagonists, there are many engaging stories that can be told even through the lens of a character without personality. Many books and works of fiction have a protagonist without clearly defined personalities or even clearly defined characters at all, but they do not seem to do worse commercially. Also, since this is a narrative-driven medium, it is possible for players to define their protagonist’s personality and actions and have these affect the story.
Well, some would say that it is both, and I am one of those people. It depends on if the game has a clearly-defined protagonist, but I think that CS games tend towards the former. CS games feature choice as a main component, so simply experiencing a character’s life is not as engaging. The ability to choose makes up a lot of the appeal in these games.
Also, I agree with the people above that the thread title seems to be a false dichotomy. The two are not mutually exclusive, after all.