Ultrasound vs x ray, which is better/more efficient?

Depends: It depends on the problem you are looking for. General rule, x-ray is to eval bones and u/s is to eval soft tissue and solid vs fluid.

Depends: Ultrasound and x-rays are both used to "look" into the body for diagnosis. X-ray uses ionizing radiation which is safe in doses given for usual images, ultrasound uses high frequency sound. Each modality has its strengths and weaknesses depending on the part of the body being imaged or the disease process the doctor is looking for.