Hi there,
The mode that is the best is the one that allows the greatest screen distance
between the reference phase and the input phase.
Since the x y mode is limited to phases that are relatively large and it is
also very hard to judge small angles on screen, and the dual beam can spread
the phase difference out along the screen x axis even for small phase angles,
I'd go with the dual beam method.
Think about it this way:
How would you measure a phase angle of 0.1 degrees using the x y method?
How would you measure same with a dual beam?