The Light in the Piazza has a bold, strong, and incredibly original score. Guettel has a completely unique voice for a musical. It's a cohesive, coherent, and deeply meaningful work of art. The characters are so strong and real that they live outside of the performances being given, and all of the characters are developed rather than two.
There are incidental similiarities between the two, but Piazza is in a league of its own, whereas Grey Gardens is a fun curiosity, awards aside. I don't know why people keep comparing the two shows. They aren't similiar except in superficial ways.
It's interesting that Caroline, or Change, Grey Gardens, and Piazza are to varying degrees and for various reasons very much about the relationships between a mother and her daughter. Caroline, or Change is by far the most ambitious (though not my favorite). It was devastating in the theater but I never listen to the cast recording. Go figure.
yr ronin,
joey