I have a couple of tanks where I use an AC HOB filter in conjunction with a canister filter. Because the Aquaclears flow so much water and are so much easier so clean than a canister, I like to dedicate the AC to doing the "heavy lifting" mechanically and use double sponges or sponge/carbon combo (easier to remove carbon from AC than a canister if I need to medicate). I think that sponges will do just as good a job as bio balls will in terms or harboring bacteria with the added benefit of mech filtration. Just remember to rinse them in tank/dechlorinated water when you clean them to preserve bacteria.
The same can be said about any other additive (crushed coral for hard water tanks or peat pellets for blackwater fish)... I believe you get the benefits of the bio balls (biogical surface area) while also serving other purposes. In this case, I say stick them in the canister if you have room.
If you're set on using a media dedicated to biological surface area, I'd say go with ceramic rings instead of bioballs as they take up less space. Bioballs are best in sumps or giant canisters imo.