I can say that 10 years go, I would have chosen M$SQL over MySQL and it would have been the correct choice. At the time I had almost 10 years experience with M$SQL and almost none with MySQL. Now I have more than 10 years of MySQL under my belt. AND the MySQL experience is more current. Right now I could choose between the two based on specific features and performance characteristics. For me to pick Posgresql because of a specific feature would be insane because I don't experience with it. No knock on Posgresql ... maybe I'll spend time with it and pick it for some down-the-road project.
I have implemented couchdb as a caching solution. I know how to manage, backup, and restore the database server. I have managed a 5 node cluster. If you ask me to implement NoSQL, it would be my choice for 2 reasons: 1. It can do the job. 2. I have experience making it do the job.
I'm sure there are 10 million people out there would would choose mongo in the same situation. The would not be wrong and they may come up with a superior solution. For me to implement Mongo today would be wrong - I would almost certainly come up with an inferior solution. for them, it would be stupid not to.
I'm not saying "don't learn anything new". I'm saying "don't gamble your business on technology with which you're not familiar".
Its a bit like backups ... the most important thing about a backup is not the technology you use, but whether you are capable of restoring and maintaining the backups.