Both depending on the workload. Usually redis is pretty good at in-memory applications and can do key-based persistence. Postgres is more made for SQL and can do key value persistence as well. I would rather choose PostgreSQL, with a bit of redis for specific parts.

Probably both. They're apples and oranges.

PostgreSQL is ideal for storing long-term non-volatile data items, like your stock, users, orders and so on.

Redis is ideal for volatile/cache storage, for things like session and transient message queues.

If you're building an enterprise-grade web based app, you will probably end up using both.