I use both Emperors and Aquaclears and as far as HOBs go they are the best. If noise is a concern, I can tell you that the Aquaclears are definitely more quiet. TCTfish is right about the Aquaclear being more customizable. I always just ditch the carbon bag and run them with 2 sponges stacked on top of each other with the bag of ceramic rings on top. The Emperors are not as customizable, but they do come with an extra media basket (1 for the 280, 2 for the 400) that I always just put filter floss in. The Biowheel is nice IMO, but it will be more expensive in the long run to use an Emperor because of the filter cartridges you have to keep replacing. The Aquaclears don't use cartridges at all, just the sponge blocks, so you can just rinse out the sponges over and over again (in tankwater ONLY) until they start to break down. I have some that have had the same sponges for a couple of years now. Plus you can get the filter itself cheaper if you choose the Aquaclear, but like I said you can't go wrong either way. What size tank do you have, and what specific models are you looking at?