Unless you are going for something exotic (say Eriocaulons) most every plant is going to like harder water for the nutrients.
I was watching a DVD of one of the AGA conventions where Claus Christensen was talking about exploring aquatic plants in the Mato Grosso, Brazil. There he notes that most of the water is fed from natural springs and is very hard (above 10 dKh/Gh). He also notes that some few pools are rain fed only, and very soft. The spring fed pools have a greater variety and more robust growth of plants in general. He says that while some plants may live and do ok in the softer water, mostly due to lack of competition, that the hard water is where you find the most robust examples of any given species.