For a betta in that size tank, I actually like the tetra 3i filter best. It doesn't create enough current to push them around.
And while a sunsun mini canister might add to overall volume, the flow will likely be too much for a betta in this size tank. If the choice is only between that and a sponge, I'd go with the sponge. I've never had a problem with any of my bettas and a sponge filter.
You'll either want one of those tiny air pumps from the 1 gallon petco kits, or the smallest pump you can find with a flow valve turned way down.