I like Postgres, and I like MongoDB too. Both have their strengths. Best tool for the job I say.
The great thing about MongoDB is you can install two or three servers in different datacenters, and have redundancy out of the box. It's really simple. And you can scale horizontally if you need to without any downtime.
The last time I looked at Postgres, to do the same, you had to use third party solutions, and the client side drivers didn't support it. Is it any better now?