Moved to equipment/hardware forum:
Either would be fine for a 20
gal IMO. The CSS is HUGE so you may not want that big of a skimmer hanging off your tank (or the large pump in the tank). I assume you're looking for a
HOB model and have no plans for a sump.
If planing on a larger tank in the future I'd get a
Aqua C Remora pro so you don't have to spend money again on a larger skimmer.