Since you directly pull JSON objects out of MongoDB (i.e. usable data structures), MongoDB lends itself to a slightly different programming style than you'd have with a relational database and an ORM. I'd hesitate to call either one more idiomatic in a lisp-ish programming style.

EDIT: That said, MongoDB's reliability model (you have to have a synchronized pair of DB servers to get the reliability you get from a single PostgreSQL instance), PostgreSQL may be the safer choice.