Definitely 20 long--the composition for planting is way better, and you don't need a very strong light at all to grow plants well because of its shallower depth. A basic fluorescent light fixture with other indirect fluorescent light should be plenty. I would use a canister filter, because they tend to be more customizable and higher quality--you can also control the direction and flow of the outlet easily.
As far as stocking is concerned, I'd pick a lovely HMPK in a bright, contrasting pattern. Oh, since we're dreaming, let's say... someone like him:
And then I'd use bright green plants like water wisteria and lloydiella to make his colors really pop, along with dark cryptocoryne wendtii, red nesea, and dwarf lilies for contrast. I would use a dark planting substrate like EcoComplete or Flourite black sand if I wanted a lot of bottom dwellers. Catfish love to dig around in sand.
Additional livestock would be a school of celestial pearl danios, because they're tiny and lovely:
And a school of kuhli loaches. Because watching them interact with each other and their environment is very entertaining:
A mix of pygmy cories and panda cories would be great for making a small tank like a 20G look bigger, though, and not everyone is into the snake-like quality of kuhlis. Since pygmies and pandas are different species, get enough for each to have their own school to swim with.