HOB vs Sponge Filter?

What do you have in the tank?

If you want to keep the hob you can always cover the intake.

Why not the best of both? Add a sponge to the intake of your HOB filter. It'll serve a similar purpose of giving more surface area for bacteria, provide pretty good mechanical filtration, and still have higher flow rate than a sponge filter.

I run a small power filter and a HOB. Never hurts to have 2 in case one fails. The powerhead is about 2.5 inches square and the filter section is about the same size using just a sponge. It doesn't seem to take up much room and hides well behind a fake log and a sword plant. It is adjustable low enough that it doesn't create a whirlpool in the 10 gallon (though turned up all the way is a little extreme) It also has a port on the outlet for a 1/4" hose. It came with a piece designed to vent to the top of the tank to add air bubbles into the outlet stream, but I use it to inject my DIY CO2.

All that said, you are not going to get more bacteria than you need. So if your current filter is able to keep up with the bioload, adding another won't do anything for bio filtration. So it comes down to what you are trying to accomplish and on what budget.

-keep stuff out of the intake? add a sponge to the intake/cheap
-add bio filtration? only if the current filter can't keep up will another one help/likely not needed
-failsafe or secondary filtration? always a good idea but not necessary/relatively more money

Another aspect of a second filter is you can use it for a quarantine tank or to start off a new tank. Just be aware when you remove it from the main tank, the primary filter for that tank will have a mini cycle.