Keep it simple.
People don't visit a website to admire the authentication.
I made that mistake myself a couple of times. (and I keep telling myself to forget about the authentication)
It's statements like "you can only make 1 first impression" and "it has to be perfect",
that subconsiously pushes developers to a waterfall approach.
All startups start with a proof-of-concept, try things, fail, retry.
And users don't mind. They really appreciate you're trying different things.
Last time I checked, even amazon doesn't allow users to register using their google or facebook account.
That doesn't seem to hold them back.
So, really whatever takes the least effort and then come back to it later.
