For same price the Grand is nicer than the Regency. But seldom are they the same price.
The Grand is accessed from inside Terminal D, but outside of security, which might be an advantage worth paying for if you were arriving and leaving from Terminal D. The Regency is a stand-alone building landside of Terminal C, but, to the best of my knowledge, no longer connected by a walkway from the terminal (but check the Hyatt site for directions). Now requires a shuttle ride, or knowing how to cut through a parking garage (including up and down stairs).