Dwarf Hairgrass - Eleocharis Acicularis, Parvula and Belem (sp. mini) - grows 2-3” tall
Mini (Eleocharis sp. “Mini”, aka Eleocharis Belem) - grows ½ -1 ½" tall
Acicularis grows straight toward the light up to 3” tall. Parvula leaf blade is like a straight sword growing upward up to 2” tall. Belem will start to arch toward the ground up to 1 ½” tall.
I've grown both and prefer the "Mini" simply because it grows no more than about 3/4" in my tank (higher light shortens maximum growth height). So, I don't have to trim it at all. When I had the taller version, I had to trim it weekly to satisfy my desired height as it grew to about 2".
Now, if you like the higher growth, the "Dwarf" is straight and looks much more like grass in a lawn. The "Mini" tends to bend over as it reaches max height but, from more than a few feet away, it looks like a grass carpet.