I've used both and prefer next.js. Granted I'm using GraphQL but I'd prefer it regardless.
RBP is actually more opinionated. It's "best* practices to the extreme" at the cost of productivity and lots of boilerplate. It's also a little hostile towards server-side rendering.
I enjoyed RBP and learned from it, but we switched to next.js (with styled-components and apollo-client) and things have been far smoother. To be honest I can't really think of a reason to use RBP over Next.js.
The creator @mxstbr has tweeted positively about Next.js, has other projects to look after (e.g. styled-components), and hasn't updated it since January, so I'm not sure RBP has much of a future anyway.
Very interesting approach!
What are the pros and the cons? Someone that has used both should help us here.