These are fundamentally different database technologies. Postgres is a powerful SQL based store while Mongo was originally built around the idea of a write never failing.

Postgres tends to keep a speed edge but the jsonb syntax is awkward and some operations can be trickier for a developer to navigate. Scaling uses traditional sql strategies.

Mongo has a pretty nifty query language but more difficult queries start becoming quite slow. Scaling is a bit different then Postgres.

Like any tech choice, it's not about what is vogue or trendy but what best fits your problem space.