Canister filters, which sit below or beside the tank and pump water through a sealed multi-chamber unit, generally hold far more filter media than a hang-on-back unit of comparable flow rate. That extra capacity means more room for biological media, which translates to more stable water parameters between maintenance sessions, especially valuable on heavily-stocked or larger tanks.
Hang-on-back filters, by contrast, are simpler to install and maintain — most use a cartridge or basket system that can be rinsed or swapped in under a minute, without the disassembly a canister filter's hose connections require. For tanks under 40 gallons with moderate stocking, this ease of maintenance often outweighs the media capacity difference.
The best filter is the one you'll actually maintain consistently, not the one with the best specs on paper.
Noise is a real factor rarely discussed in spec sheets. A well-maintained canister filter, with a good impeller and prime, runs nearly silent since it's sealed and often located inside a stand. A hang-on-back filter's characteristic trickle and occasional rattle from a loose intake tube is more noticeable in a bedroom or living room setup.
For tanks 40 gallons and up, or any heavily-planted or heavily-stocked tank, a canister filter's media capacity and lower noise profile generally justify the higher upfront cost. For smaller, lightly-stocked community tanks, a quality hang-on-back filter is more than adequate and considerably easier to service.
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