The one with a placeholder is "more efficient" in that it pre-plans the execution before knowing what value will be stuffed into the placeholder. Unfortunately, without that information, the resulting plan sucks. [By pre-planning at prepare() time, it doesn't have to waste time re-planning every time you execute() the same prepared query.]
I have yet to run into a situation where I had and actually kept "server-side prepare" enabled, despite lots of people wanting to crow about how prepare + placeholders can be such a performance "win".
Luckily, it is easy to disable server-side prepare so that you can still use place-holders while still getting "late" query planning every time you execute(). See pg_server_prepare since we are talking Postgresql.