Hi,
When converting SLC product into GRD, phase is not retained. If you want to perform interferometry, you should use SLC format.
GRD data has been multi-looked and projected to ground range using an Earth ellipsoid model.
Pixel values represent the intensity but phase information is lost. You can use GRD data if you are interested in using only the intensity values (e.g perform classification), on the other hand, you can use the phase information if you are interested in doing a subsidence analysis.