Pros and Cons to both, but in a NodeBB environment, both are relatively similar in the long term.
Redis stores data in RAM, but saves to disk in time/data increments.
MongoDB just stores data on disk.
I've been running 35hz.co.uk on Redis for over a year and not had a Redis related issue in all of that time. It's been nothing but stable.
I've not used MongoDB, but when your site becomes too large for Redis (cost per GB of RAM is higher than cost per GB of SSD storage), a move to MongoDB is always a wise one. (NodeBB.org uses MongoDB now, but used to use Redis (reason for the move was to test their exporter)).
Choices are entirely yours really. You'll find just as much support for both on the forums.
Well answered, that's exactly right. As an added note, our premium hosting services use both MongoDB and Redis. #magichax just for our paying clients (well okay, it's also in our docs so you can do it yourself)