according to the wiki, the plastic rectangles consume more power per node. aside from that, they both have three rgbleds that are controlled together.
the metal squares have them in a triangle shape
the plastic rectangles are in a straight line.
the plastic rectangles are about twice as thick as the metal squares, bu they are skinnier.
both are commonly used behind something to illuminate it.
for me, i used metal squares inside an acrylic globe to light up the globe.
And i used plastic rectangles to illuminate coroplast
