I personally like whisper brand. the refill catridges are really cheap and they do a great job. the penguin models are really good, but the refills are rediculous. i have an emperor 400 and the cartridges are like 10 bucks for 2 at the store. and the filter takes 2 at a time! so i had to buy them online a l;ittle cheaper, but still. i learned my lesson!
the whispers have a bio sponge, so just like the more expensive bio wheel models, you still have great bio filtration.