Penquin filters are one of my favorites. I really like the 330 because it has separate baskets that you can put extra bio-media in.
The Penquins that are smaller rely completely on the bio-wheel for the support of your bio-base. I like having the bio-wheel, but it seems to take longer to get colonized and does not hold as much bacteria as other media. I have taken apart the cartridges of the 170s and filled them with media so they contain more.
The Emperors - the next step up in the same brand are also excellent filters. There is an Emperor 280 that has a basket for biomedia. This is the equivilant to the Penquin 330 - except that it is not as wide and has only one biowheel. An Emperor 400 is a good worker too. I have one on each of my 30 gallon tanks and they have given me no problems.
I run a Penquin 330 on one 20 gallon tank and an Emperor 280 on another. They are good filters.
Since you do not need to throw away the cartridges as often as they say, they are a fine buy, also. I just shake the cartridge down really well every cleaning and put it back. I only replace it when it gets good and hairy and worn out.