The T5HO is a very bright light for a tank as short as a 20L. You would need to raise the light 12 inches off the top of the tank to get to acceptable light levels. Otherwise you will fighting alot of algae.
You could get the dual t8, and then if you ever want to upgrade to higher light get a separate single T8 at that time.
A single T5HO might be suitable as well for higher light options.
This chart might help...