Callanetics isolates muscle groups by using tiny, precise (yet powerful) movements. Using these tiny contractions to work the muscle, you get 'deeper' into the muscle to reshape and tighten without any 'bulk'. The benefit is that you are increasing strength, flexibility and body alignment while obtaining a long lean look.
Traditional weight training works large and small muscles by adding resistance to the movement. This method creates strong mucle and lean muscle mass to raise your metabolism. It will also add more visible definition.
In short, do Callanetics for a strong, lean dancer like body and traditional weight training for a more defined, athletic physique.