The built in horizontal scaling of Mongo is nice, but I would still put PG's reliability per-instance higher than Mongo's. The reason being that Mongo is writt

The built in horizontal scaling of Mongo is nice, but I would still put PG's reliability per-instance higher than Mongo's. The reason being that Mongo is written in C++ with lots of RAII and will simply die on a memory allocation failure. Postgres won't. It'll keep running in many low memory scenarios and the operation will error out.