A lot of people like AC, because it's cheap to run on reusable foam. To me, cleaning the gunk off the filthy foam is a pain, and the coarse texture of the foam does not provide good mechanical filtration. My preference is Penguin, simple design and cheapest HOBs in the market, and the thinner box design takes up less space behind the tank. Penquin provides more effective mechanical filtration using fine texture filter pads and changing the pads is a no pain 2-minute task. No, I don't buy expensive pads from the manufacturer. I buy very cheap polyester batting from fabric shop and cut my own pads at less than 10 cents per pair. I operate the Penquin straightly for mechanical and don't use the biowheels as they will slow down the flow. For the same reason, I prefer Penquin over its more complicated Emperor line as I don't care for the jet stream and biowheels.