Which one you choose does matter though, at least for the immediate user experience.

I don't think decoupling the content distribution system from the immediate interface works in the favor of large scale adoption of the whole system.

We can be honest by saying something like "the particular site you sign up for will look and feel different, and the people you first encounter will have a slightly different culture, but ultimately all of these sites can and do interact with each other's users and content. It's like choosing one of twitter or facebook as your main interface but being able to read and post on either one. It's a lot more complicated to talk about than it is to use it, really. You'll see when you try it".

Most importantly, people can start somewhere, and if they don't like the first site they choose, they can register with another. We should be finding ways to tell people this isn't the end of the world if they feel the need to go to another site to use the same overall ecosystem.