The ocean landing is more difficult. But you have to realize getting to orbit is not about height, it's about speed. By the time the first stage is separated

The ocean landing is more difficult. But you have to realize getting to orbit is not about height, it's about speed. By the time the first stage is separated the rocket is going 3000 m/sec away from the launch pad. It takes a lot of fuel to arrest that motion and go back the other way, whereas the barge will be right where you want to land given the situation at separation.