Use of different formulas for capacitor impedence (phasors)
Both these equations are correct because \$j = \sqrt{-1} \$

We have 

$$Z_{cap} = \dfrac{1}{j \ \omega \ C} = \dfrac{1}{\sqrt{-1} \ \omega \ C} = \dfrac{1}{\sqrt{-1} \ \omega \ C} \cdot \dfrac{\sqrt{-1}}{\sqrt{-1}} = - \dfrac{\sqrt{-1}}{\omega \ C} = - \dfrac{j}{\omega \ C}$$