while dual and SC are similar the core gameplay will be different, dual will have a large portion of content on building and farming materials, the reason SC is taking so long is the amount of detail going into everything, no other game comes close to the quality in everything they are putting into the game.
Dual will be a game for mostly people who like to build things, where SC is focused on the experience, will probably try dual but my main game is still going to be SC and can still play both depending on your mood.