What I really liked about building my own filter (apart from saving about 90%) is that everything is super customizable. You could use a variety of different inner media, including a small media bag with carbon if you needed it. You could also build layers inside the filter or around the sponge with those water polishing pads or whatever. Also, it is entirely feasible to support multiple tanks on a single pump (provided you have one of those with the dual connection). You can't do that with a HOB. The only downside is that I haven't figured out a way to take up less space in an aquarium. That's the benefit of a HOB. I've been looking for a way to use similar items in order to create a HOB, perhaps by finding a way to make the tubing to draw water up and into a filter situated on an exterior wall, with an output lip up at the top...