"Puller" fans are more efficient. It has more to do with the properties of air low. Your get better flow over a heat exchanger (in this case a radiator) by inducing a low pressure over the back side and drawing the air through it.
Trying to "push" the air through the front creates much more turbulence and much less actual air flow per unit of energy consumed by the fan motor.
If you look at any industrial heat exchanger, like say your home air conditioner, you will see this is how they are all designed. It's just the characteristics of fluid dynamics.
With cars though, things like proper engineering take a back seat to style and packaging.
Oh, and having the obstruction of the fan assembly in front of the radiator isn't any worse than having it behind the radiator. Wherever it is in the path of the air flow it will be a restriction.