Trying to build on persistent HTTP connections that don't close is a recipe for frustration when scaling horizontally, which is something you plan to do, since the reason why not to go with Postgres is so you can have more than one instance, right?

You can't juggle the connection between different servers. If a server drops out (because it's being scaled in), then you lose the connection, and you have to establish a new one, which introduces a hiccup/latency. No free lunches.