I'm in no way an authority on database options, but I recently made the decision between RethinkDB and MongoDB for a simple Express service. I really wanted to try Rethink, but I ultimately chose Mongo. The decision came down to the fact that I could find a reliable, free option for deploying MongoDB (mLab).
While Rethink and it's principals are appealing and seem to be on the rise, Mongo is already well established. I'm more experienced with Mongo, and the community as a whole seems to be as well. In that way, I would agree that Mongo seems to be the safer option.
Popularity alone doesn't make Mongo the better option, but for a junior developer like me, the most popular choice is often the correct one.