The Stampede or Dart 20 is indeed just a big Dart 18. It's a great family boat since it's boomless and will carry a good bit of weight. The skegs (not boards or asymetrical hulls) work well. While you can sail it single handed in moderate breeze... it's a bit much to move on the beach by yourself.

Boomless works for the family.

Dart's are still made in england and you should be able to get parts, sails, or rudder rebuild kits when needed... Sol cats are long gone and you would have to retrofit parts and replacement sails.

the Nacra 5.7 is a similar design... boomless and has a skeg. Neither boat is a race boat at it's core.

Go with the big Dart!