National Rail, London Stansted Airport - Liverpool Street: 47 mins. Off peak return (valid for 1 month): £34
National Rail, London Southend Airport - Liverpool Street: 53 mins. Off peak return (valid for month): £23
So, it takes 5 mins longer on the train, but the journey from aircraft to train is considerably less at SEN than STN, and at SEN the train is £11 cheaper …
Remind me again why SEN is not a good London airport?