I have Lego Star Wars (LSW) 1 and 2 for the GameCube and I recently caved and picked up LSW:CS for the Wii. *I* think there's enough new content to justify spending $20. (but I may just be bored waiting for LSW3) They changed two or three levels in LSW1 to made them a lot easier. They added some new touches to levels in LSW1. There are a few new characters to use in Free Play.
The biggest change is the "hub world". LSW1 used the diner as a "hub" world. LSW2 used the Mos Eisley Cantina. LSW:CS uses a totally new "Cantina". The way you view unplayable minikit vehicles is different. They're still just sitting outside the "hub" world, but now you cycle through them instead of walking around.
An additional set of blue minikits exist in every level (accessed via playing the level as Challenge) for added replay value.
(I'm anxiously awaiting LSW3, and I have a feeling they'll try to grab more cash by bundling all three LSW games in the future. That's the LucasArts track record.)