The advantages canister have over power filters is that they hold more media, and the media can be customized more easily. More media means you can have more efficient filtration with less water flow, and get by with less frequent maintenance. Customization would be the ability to set it up as mainly a mechanical filter, mainly biological, or adding something like peat to lower the pH.
The ac 20 would be fine for an 18 gallon with less messy fish, or less heavily stocked. The advantage the ac's have over most other power filters is that they can be customized similar to a canister, due to the fact that the media chamber is open with a large clip, not dependant on pre-made cartriges like many other power filters. On my 20 gallon tanks I have them set up as mostly mechanical with 1 or 2 sponges, and run a large sponge filter as well for bio filtration. A couple have the bio media that came with them, I tossed that in there, figured it won't hurt anything.
3 of the 20's got upgraded to 29's a couple of months ago, and got stocked the way you would expect a breeding situation to be stocked, pretty well overstocked. The 20's weren't keeping up, needed to be cleaned more often than once a week, so I upgraded to the ac 50's.
I was cleaning them around every other week, the gradual 50% stocking increase was killing them, so I knew it was time for an upgrade. The 50's will run for over 3 weeks before slowing down, I always give new filters the torture test, just in case I need to use that warranty.
I would go with the larger filter.
The newer ac's come with media, a sponge, carbon, & the bio media. I rarely run carbon, A good setup would be a sponge, some filter floss, & the bio media. The floss will have to be changed occasionally, those sponges last forever, I have some that are going on 5 years old. The ceramic bio media should last indefinitly.
