Right query to get the current number of connections in a PostgreSQL DB

Those two requires aren't equivalent. The equivalent version of the first one would be:

SELECT sum(numbackends) FROM pg_stat_database;

In that case, I would expect that version to be slightly faster than the second one, simply because it has fewer rows to count. But you are not likely going to be able to measure a difference.

Both queries are based on exactly the same data, so they will be equally accurate.

The following query is very helpful

select  * from
(select count(*) used from pg_stat_activity) q1,
(select setting::int res_for_super from pg_settings where name=$$superuser_reserved_connections$$) q2,
(select setting::int max_conn from pg_settings where name=$$max_connections$$) q3;

They definitely may give different results. The better one is

select count(*) from pg_stat_activity;

It's because it includes connections to WAL sender processes which are treated as regular connections and count towards max_connections.

See max_wal_senders