The more filtration, the better. Sometimes it also depends on your stock as well. If you have a good right amount of fishload in the tank, and you dont want to put more fish, then there's really no need for the bigger if you want to save. But then again, if you actually upgrade to a bigger tank in the future, the aquaclear 50 filter will be a good investment.
Remember when starting a new tank, there is no media(good bacteria) in the tank. You have to start a cycle in the tank first before adding fish. Use Bio-spira(good bacteria), then add fish. Remember, whats also important is fish research;you would want to know the maximum size of the fish and how dirty they are and etc before buying fish. Look for fish that is suitable for 20 gallons.
Buy water test kits too. To measure pH, nitrite, nitrate, ammonia.