I have a Penguin Minis on my 10 gal
and 7 gal
. They work very well and are quiet except for the sound of trickling water. Like Alli said, the Biowheels are nice because the biological filter is physically separate from the mechanical filter.
Trouble is, the biowheel will not rotate if the mechanical filter cartridge is very dirty - resulting in a mini cycle. Cleaning it buys you a few weeks at most, then you have to replace the cartridge, which can get expensive over the long haul.
I have no experience with the Aquaclears, but lots of people here swear by them.