AC by a wide margin. The way the Penguins return at surface is noisy. AC seems to stir the tank more.
Interestingly, no one mentioned Tetra Whisper filters which is one of the three main line HOBs in the market. I retired all my old Whispers because they are not reliable in restarting in power surge. Not sure if the newer Whispers have improved.
If you replace the filter pad frequently, Penquin is very quiet. The water cascading noise comes from too low tank level but more likely from clogged up filter that forces the water to break out from top. All HOBs have much higher flow rate relative to the filter capacity, so they break out quickly. I have to change my Penquin filter pads about twice a week as soon as I hear the break out noise. AC uses foam block that has large pore spaces, so it doesn't get break out as quickly and stirs stronger. But the trade off is that AC doesn't provide as good mechanical filtration letting fine particles to pass through. I used to run AC and Penquin side by side and can see the difference in water clarity.
I use a sponge filter and an AC 50 on my 45 gallon tank. I love ACs great filters, very versatile and reliable. Sponge for bio AC for mechanical.
That's a bit if a stretch, but they do last awhile. Every time you wring them out during cleaning you're causing wear and tear. Ultimately, they will fall apart. I replace mine after a couple of years.
One reason AC is so popular is that you don't have to replace the filter pad each time you clean it as the sponge is reusable and lasts for a long time, but not forever though. But I found a cheap source of material to replace my Penquin filter pads from craft shop that costs next to nothing, so I don't mind replacing them often.
http://www.joann.com/comfortloft-10....html#q=polyester+batting+by+the+yard&start=1
http://www.joann.com/comfortloft-10....html#q=polyester+batting+by+the+yard&start=1
