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.