I would vote a skimmer. They hang on the back as well. They cost a lot more then a HOB, but they do a better job taking gunk out of the water.
If you can't afford a skimmer, then a regular biowheel (like penguin - which is what I have and haven't had any problems) would work. Just make sure to get one that is rated ABOVE your tank size, so if you have a 30 gallon, get one that is rated for a 50 gallon - the more filtration the better IMHO.