Your 20g idea is better. Just put plenty of live rock into your tank (60-80lbs), and invest in a good skimmer (Beckett, Downdraft, induction, or Recirculating preferably) and you'll have a nice filtration setup.
You sound like a beginner to the hobby, I wouldn't recomend a calcium reactor. They are very expensive, complicated, and are of no use to anyone running a 55g unless you are keeping a very high-calcium-demand system (which you'd only have if you are keeping corals that beginners shouldn't keep).
Good skimmer brands to consider are AquaC, My Reef Creations, Euro Reef, etc. Lots of people use Coralife skimmers, but I myself do not care for them.