Correct query to check streaming replication lag

Hello All,

Can anyone please tell me which of the following is the correct replication lag query to find streaming replication lag in seconds?

1. SELECT extract(seconds from (now() - pg_last_xact_replay_timestamp())) AS time_lag;

2. SELECT CASE WHEN pg_last_xlog_receive_location() = pg_last_xlog_replay_location() THEN 0 ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()) END AS log_delay;