imo, tunze are more stable and have better longevity than both koralia and vortec powerheads, and are priced less than vortec but more than koralia's.
i like the controlability and the back up battery for the vortec, but the parts in the tank are prone to failing and needing replacing every year or 2, and for ther cost this seems excessive.
and koralia just dont have the build quality or customer service that roger at tunze usa provides for his customers, plus, i think the tunze powerheads look a little nicer in the tank if you cant hide them behind your rockwerk, and are quieter than koralia powerheads. plus koralias have been known to put voltage in the tank if they fail/malfunction.
also, tunze offers the same controlability with their med. and large powerheads, and you could easily run them on a computer battery back up for a long period of time if the power went out.
for a 55g, i would get 2 tunze 6045's for a total of about 2,400gph it would run you about $200. then set them up in a gyre configuration for more laminar flow, which has been proven to be more benificial than randomizing your flow patterns. i do like setting up 2gyres for a tidal reverse of the flow every 12 hours, but that means double the cost and double the powerheads.
http://www.youtube.com/watch?v=Mz15R0gVMPA