You get what you pay for. With the Tunze you get proven powerheads that are completly controllable flow. The Tunzes have been proven to last a long time (with proper maintance) they are very reliable and have protection built into them. Plus you cannot beat the output they put out.
I'm designing my 120G tank now and I was having a real hard time sucking down the huge cost of the Tunze systems. But after adding up the cost to get similar water flow with the variable output the Tunze system wasn't really anymore expensive. Now this was comparing a Closed loop system vs. the Tunze, but flow is flow. You spend so much money making your tank a piece of art, reliable, and a place for your coral and fish to thrive in. I just could not see putting in cheap unrelibale powerheads in there. But that's just me...