Based on the requirements (i.e. acceptance criteria, pros and cons of weld flaws/imperfections, type of weld, thickness of plates, recordkeeping of test results, cost and time schedule etc ) one has to take a call for appropriate NDT method.
UT and RT have their distinct advantages and the method which is best suited should be used. Both tests are good enough to detect volumetric discontinuities.
Though State-of-art scenario today is still in favor of RT; Ultrasound Test is commonly used for testing thicker sections accessible from one side only, and can often detect finer lines or planar defects which may not be as readily detected by radiographic testing.
(RT cannot detect discontinuities that are located perpendicular to the rays & UT cannot detect discontinuities that are parallel to the ultrasonic beam)
Cheers! -VH