Of Course Dance central is better than Just Dance. Just Dance only detects where the Wii waggle controllers are. It has no idea how you are actually dancing. You can sit on your couch and just flick the Wii controller around and do well in Just Dance...without dancing at all or even standing up.
With Dance Central, the Kinect unit detects every part of your skeletal body, your arms, legs, joints, etc. so knows exactly how you are dancing. It even will show the part of your body that isn't dancing correctly by marking it in red, so the game knows if you are dancing with proper form, etc. It is light years ahead of Just Dance.
Plus it is made by the Harmoniox guys, the guys who make Rock Band, so the quality will be through the roof. And every week there will be new downloadable songs so the library of songs you can dance to will just grow and grow.
BTW, the only reason Dance Central only supports one dancer at a time right now was due to time constraints. Kinect can definitely handle more than one dancer at a time if they have time to program it. I am sure Dance Central 2 will support two dancers at a time, maybe even three or four.