I think SSO option is becoming de facto standard.
IMHO, this post: “Single Sign-On: The One-Click Growth Hack You’re Not Using” explains it very well.
As a regular user, I always use SSO. Most apps offer other option as well. This gives user flexibility to choose which one fits best.
Coggle is good example for register without setting password. Coggle doesn’t offer any option but Sign in with Google
Sign In Page
For example I share images of Coggle example. Why did I do this? Because I know that mobile users don’t like shifting between websites and They don’t like filling up form as well.
Luke Wroblewski, author of Web Form Design: Filling in the Blanks book, also have same opinion.
By the way, this video is part of Building High Conversion Web Forms course. (It’s free)
But I wouldn’t just offer SSO to user. In fact there’s already a pattern. Let’s check two websites all together.
Creating an experience similar to these examples will also support consistency and standards which’s one of 10 Usability Heuristics for User Interface Design.
Lastly, there’s one slight difference between two signup form above. Datacamp lets you sing up even with signin button however, Udacity don’t. I think Datacamp’s approach is better.