If you're trying to cool the whole house down using several window air conditioners then window ac's would be less efficient than central air conditioning.
If you're just trying to cool one or two rooms then the window ac's would be much cheaper and more energy efficient than the central air conditioner.
It could cost $300.00 or more per month for the central air conditioner to cool a whole house down during the summer but if you stick to cooling just a couple of rooms with window air conditioners than your cost could be as low $50.00 per month using the window air conditioners.