You can totally use a cheaper hang on back filter no problem, but I think you will appreciate the flow pattern and filtration capacity the canister filter offers.
For a 20 gallon community tank, I highly recommend a small canister filter like the Oase filtosmart thermo 100 or the fluval 107. You can totally use a cheaper hang on back filter no problem, but I think you will appreciate the flow pattern and filtration capacity the canister filter offers. Don't over think this component, just search out a filter that's flow rate is recommended for your size of tank (on the box of all filters). If your tank is larger than 35 gallons I highly recommend a canister filter over any type of internal or hang on the back filtration.