Beags,
The E-3D has both refuelling systems by default - it was considered too expensive and impractical to remove the Boom receptical from the air-frame. Since then, the USAF boom receptical has been the most used by far. I understand from the flight-deck types tha,t not only is the US system much faster to transfer fuel but, is also simpler in operation. We also found that there were many more Boom tankers in theatre than drogue draggers.
So, although the probe was occassionally used for a spot of heavyweight jousting, I am sure that if it was the case of one vs the other, then the receptical wins hand down - it is also cheaper! After all, we do not want another Nimrod type lash-up on the aircraft do we?