I sailed and raced a Stampede (dart 20) a number of times.
I Loved the dart 18 but think the 20 is not so good. The 20 has all the sail area of a 20 ft catamaran, but none of the sail controls that make things managable in high winds. Ie no good mast rotation control, no good downhaul etc. The boat is a handfull. Because of this it takes 2 large males to make it go in a blow. I would go with a nacra 5.0 or 5.7 if possible for that reason. If you don't want daggerboards.
On the otherhand a dart 18 is a great singlehanded cat.
Eric