Petrol vs Diesel in my Seat Ateca - which one is more economical?

I'm currently debating whether to stick with petrol or switch to diesel in my Seat Ateca. I've had the car for a few months now and have been averaging around 40mpg on long motorway runs, but I've heard that diesel can be more economical overall. Has anyone else on here made the switch or is petrol the way forward?