The advantage of an untrasound over an x-ray is: Ultrasound's can see soft tissue, and other organs, and structures such as tendons.
Ultrasounds:
With a untrasound is used to see internal body structures such as tendons, muscles, joints, blood vessels, and internal organs. Which is very important incase of an emergancy.
X-ray's:
A X-ray macine takes pictures of the bones to see if there's any breakage to the bones, which determins of the person needs surgury to fix the breakage to the bones.