Go with the Aquaclear 200. I thing the model is a 200 for the 20 gallon tank. Been a while since I had 20 gallon tanks on outside filters. My second choice would be what I normally use for 20 gallons, a sponge filter. Hydrosponge V. A spnge filter and not very asthetic, but is works the best.
for a tank that small, i would go internal or HOB. Although, if you were to get an eheim 2213 canister, it would completely dominate in the filtration department...