I have 2 of the center units (Wave2K) in my 180. Excellent. The guy who invented them and makes them is extremely helpfull if you have any issues. Personnaly I have had to do maint on one, but they do an excellent job of flow and water movement. I no longer have to have any annoying power heads in my tank.
For a 55gal, 1 unit would be simply great. They have corner units as well if you want to go that route. As far as the tunze, I have heard great things about them. I almost went that route but it was a little too pricey for me.