Understanding equation for the quality factor of an RLC circuit?
You have confused things by equating \$X_C\$ with \$\dfrac{1}{j\omega C}\$. 

When talking about Q factors, \$X_C\$ is a magnitude with no sign hence it shouldn't be matched up with the reactive impedance of a capacitor because of "j". So, dump the "j" part and you get the right answer.