For a 20 gallon, I'd recomend to discard the saltwater option.
For what I've learned, saltwater chemistry is more difficult to mantain that of a freswater tank. Having this small tank it would be rather fast changing, and due to the cost of the equipment and price of the fishes, I's rather use at least a 50-70 gallon tank for marine.
Those were my two cents. Let ne know if I'm wrong. Thanks.