Whether you will save money depends on several factors. total wattage of the two HOBs versus one cannister will tell you which will use less electricity. The elements in the filter and whether they need to be replaced is another factor. With an HOB such as an AquaClear, the foam inserts last for many years so there is no ongoing cost for replacement sponges. Personally I find cannisters to be a pain to clean compared to an AC. However, cannisters can be a good option if you need to hide the filter, or move a tank closer to a wall.
To me, the best filtration is that which you will keep maintained. An AC filter is so easy to clean you can do it with weekly water changes. Getting solid material out of the tank is a better option than hiding it inside a cannister.
